Java二叉树进阶面试题讲解

短命女 2024-04-28 07:37 40阅读 0赞

#### Java二叉树进阶面试题讲解 ####

*  ?1.二叉树的构建及遍历?
 *  ?2.二叉树的分层遍历?
 *  ?3.给定一个二叉树, 找到该树中两个指定节点的最近公共祖先?
 *  ?4.二叉树搜索树转换成排序双向链表?
 *  ?5.根据一棵树的前序遍历与中序遍历构造二叉树?
 *  ?6.根据一棵树的中序遍历与后序遍历构造二叉树?
 *  ?7.二叉树创建字符串?

**大家好，我是晓星航。今天为大家带来的是 Java二叉树进阶面试题讲解 的讲解！?**

## ?1.二叉树的构建及遍历? ##

二叉树的构建及遍历。[OJ链接][OJ]

![202303282037607.png][] ![202303282038282.png][]

示例图解：

![202303282038978.png][]

import java.util.*;
    class TreeNode {
        
        public char val;
        public TreeNode left;
        public TreeNode right;
        public TreeNode(char val) {
        
            this.val = val;
        }
    }
    // 注意类名必须为 Main, 不要有任何 package xxx 信息
    public class Main {
        
        public static int i = 0;
        public static TreeNode createTree(String str) {
        
            TreeNode root = null;
            if (str.charAt(i) != '#') {
        
                root = new TreeNode(str.charAt(i));
                i++;
                root.left = createTree(str);
                root.right = createTree(str);
            } else {
        
                //遇到# 就是空树
                i++;
            }
            return root;
        }
        public static void inorder(TreeNode root) {
        
            if (root == null) {
        
                return;
            }
            inorder(root.left);
            System.out.print(root.val + " ");
            inorder(root.right);
        }
        public static void main(String[] args) {
        
            Scanner in = new Scanner(System.in);
            // 注意 hasNext 和 hasNextLine 的区别
            while (in.hasNextLine()) {
         // 注意 while 处理多个 case
                String str = in.nextLine();
                TreeNode root = createTree(str);
                inorder(root);
            }
        }
    }

思路：根据题目意思：我们的\#是null即为空结点的意思，因此我们再使用str.CharAt(i)来遍历我们输入的每一个字符时遇到\#就直接i++，即使这一个结点的左结点置为空，如果继续遇到\#就继续i++使其右节点也为空，然后返回我们的上一个结点。直到遍历完整个字符串我们的树便算是创建完毕了。

![202303282038091.png][]

## ?2.二叉树的分层遍历? ##

二叉树的分层遍历 [OJ链接][OJ 1]

![202303282039347.png][] ![202303282039394.png][]

/**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode() {}
     *     TreeNode(int val) { this.val = val; }
     *     TreeNode(int val, TreeNode left, TreeNode right) {
     *         this.val = val;
     *         this.left = left;
     *         this.right = right;
     *     }
     * }
     */
    class Solution {
        
        public List<List<Integer>> levelOrder(TreeNode root) {
        
                    List<List<Integer>> ret = new ArrayList<>();
            if (root == null) {
        
                return ret;
            }
            Queue<TreeNode> queue = new LinkedList<>();
            queue.offer(root);
            while (!queue.isEmpty()) {
        
                int size = queue.size();//这个值代表当前行有多少个结点
                List<Integer> list = new ArrayList<>();
                while (size != 0) {
        
                    TreeNode cur = queue.poll();
                    list.add(cur.val);
                    if (cur.left != null) {
        
                        queue.offer(cur.left);
                    }
                    if (cur.right != null) {
        
                        queue.offer(cur.right);
                    }
                    size--;
                }
                ret.add(list);
            }
            return ret;
        }
    }

思路：首先判断树是否为空，为空直接返回链表对象ret，不为空继续往下走，将root根结点添加进入queue队列，并每次拿出一个元素给cur，将cur的值添加到list中，并访问cur的左右结点继续循环直至root树为空，最后返回我们的ret即可。

![202303282039762.png][]

## ?3.给定一个二叉树, 找到该树中两个指定节点的最近公共祖先? ##

给定一个二叉树, 找到该树中两个指定节点的最近公共祖先 。[OJ链接][OJ 2]

![202303282039800.png][] ![202303282039811.png][] ![202303282039143.png][]

二叉搜索树：根的左边比根小，根的右边比根大 中序遍历的大小是有序的。

思路一：以二叉搜索树为例来讲解此题

1、root == p || root == q 此时的最近公共祖先是root

2、p.val < root.val || q.val < root.val p和q都在root的左子树 最近公共祖先在root的左树当中

3、p.val > root.val || q.val > root.val p和q都在root的右子树 最近公共祖先在root的右树当中

4、p.val > root.val && q.val < root.val q和p分别在root的左子树和右子树当中 最近公共祖先就是root

三种情况的图解：

![202303282040515.png][]

/**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    class Solution {
        
        public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        
            if (root == null) {
        
                return null;
            }
            if (root == p || root == q) {
        
                return root;
            }
            TreeNode leftT = lowestCommonAncestor(root.left,p,q);
            TreeNode rightT = lowestCommonAncestor(root.right,p,q);
            if (leftT != null && rightT != null) {
        
                return root;
            } else if(leftT != null) {
        
                return leftT;
            } else if(rightT != null) {
        
                return rightT;
            } else {
        
                return null;
            }
        }
    }

![202303282040214.png][]

当root为3，p为6，q为4时，我们代码运行的逻辑图解：

![202303282040330.png][]

> 注：每一次return是返回上一个函数的值，直到第一个递归函数return才是返回我们程序的值。

![202303282041110.png][]

思路二：假设 这棵二叉树 是使用孩子双亲表示法 表示的

1、用两个栈 存储 路径 — 如何找到从根结点到指定结点的路径

2、求栈的大小

3、计算出两个栈中 多的元素 出差值个元素

4、开始出栈 直到栈顶元素相同 此时就是公共祖先

/**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    class Solution {
        
    
        //root:根节点    node:指定的节点  stack:存放指定节点的路径
        public boolean getPath (TreeNode root,TreeNode node,Stack<TreeNode> stack) {
        
            if(root == null || node == null) {
        
                return false;
            }
            stack.push(root);
            if(root == node) {
        
                return true;
            }
            boolean flg = getPath(root.left,node,stack);
            if(flg == true) {
        
                return true;
            }
            flg = getPath(root.right,node,stack);
            if(flg == true) {
        
                return true;
            }
            stack.pop();
            return false;
        }
        public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        
            if(root == null) {
        
                return null;
            }
            Stack<TreeNode> stack1 = new Stack<>();
            getPath(root,p,stack1);
            Stack<TreeNode> stack2 = new Stack<>();
            getPath(root,q,stack2);
            int size1 = stack1.size();
            int size2 = stack2.size();
            if (size1 > size2) {
        
                int size = size1 - size2;
                while (size != 0) {
        
                    //出第一个栈里面的元素
                    stack1.pop();
                    size--;
                }
                while(!stack1.isEmpty() && !stack2.isEmpty()) {
        
                    //判断地址
                    if (stack1.peek() == stack2.peek()) {
        
                        return stack1.pop();
                    } else {
        
                        stack1.pop();
                        stack2.pop();
                    }
                }
            } else {
        
            int size = size2 - size1;
                while (size != 0) {
        
                    stack2.pop();
                    size--;
                }
                while(!stack1.isEmpty() && !stack2.isEmpty()) {
        
                    //判断地址
                    if (stack1.peek() == stack2.peek()) {
        
                        return stack1.pop();
                    } else {
        
                        stack1.pop();
                        stack2.pop();
                    }
                }
            }
    
            return null;
        }
    }

![202303282041429.png][] ![202303282047890.png][]

## ?4.二叉树搜索树转换成排序双向链表? ##

二叉树搜索树转换成排序双向链表[OJ链接][OJ 3]

![202303282042759.png][] ![202303282042152.png][] ![202303282042971.png][]

思考问题：

1.排序：可以中序遍历这棵二叉搜索树

2.双向链表：如何构建前驱和后续结点

思路：left变为双向链表的前驱

right变为双向链表的后继

/**
    public class TreeNode {
        int val = 0;
        TreeNode left = null;
        TreeNode right = null;
    
        public TreeNode(int val) {
            this.val = val;
    
        }
    
    }
    */
    public class Solution {
        
        TreeNode prev = null;
        public void inorder (TreeNode pCur) {
        
            if (pCur == null) {
        
                return;
            }
            inorder(pCur.left);//左
    
            //先判断左 给左节点赋关系
            pCur.left = prev;
            //在左走完以后 判断prev是否为空 并给右节点赋关系
            if (prev != null) {
        
                prev.right = pCur;
            }
            //在这个节点的左右关系都确定好后将prev变成pCur
            prev = pCur;
    //      System.out.print(pCur.val + " ");
            //然后开始进入右节点递归
            inorder(pCur.right);//右
        }
        public TreeNode Convert(TreeNode pRootOfTree) {
        
            if (pRootOfTree == null) {
        
                return null;
            }
            inorder(pRootOfTree);
            TreeNode head = pRootOfTree;
            //这个while的作用是找到这棵树的最下的左节点 这个左节点就是我们需要找到的节点
            while (head.left != null) {
        
                head = head.left;
            }
            return head;
        }
    }

![202303282043745.png][]

## ?5.根据一棵树的前序遍历与中序遍历构造二叉树? ##

根据一棵树的前序遍历与中序遍历构造二叉树 [OJ链接][OJ 4]

![202303282043930.png][] ![202303282043657.png][] ![202303282043833.png][] ![202303282044198.png][]

思路：

1、先将pi下标的 元素 创建为root

2、在中序遍历的数组当中，找到当前pi下标的元素，存在的位置。ri

3、root.left = ri - 1;

root.right = ri + 1;

/**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode() {}
     *     TreeNode(int val) { this.val = val; }
     *     TreeNode(int val, TreeNode left, TreeNode right) {
     *         this.val = val;
     *         this.left = left;
     *         this.right = right;
     *     }
     * }
     */
    class Solution {
        
        public int preIndex = 0;
        public TreeNode creatTreeByPandI(int[] preorder,int[] inorder, int inbegin, int inend) {
        
            if(inbegin > inend) {
        
                //如果满足这个条件 说明 没有左树 或者 右树了
                return null;
            }
            TreeNode root = new TreeNode(preorder[preIndex]);
            //找到根在中序遍历的位置
            int rootIndex = findIndexOfI(inorder,inbegin,inend,preorder[preIndex]);
            if (rootIndex == -1) {
        
                return null;
            }
            preIndex++;
            //分别找到左子树和右子树
            root.left = creatTreeByPandI(preorder,inorder,inbegin,rootIndex - 1);
            root.right = creatTreeByPandI(preorder,inorder,rootIndex + 1,inend);
            return  root;
        }
        private int findIndexOfI(int[] inorder, int inbegin, int inend,int key) {
        
            for (int i = inbegin; i <= inend; i++) {
        
                if (inorder[i] == key) {
        
                    return i;
                }
            }
            return -1;
        }
        public TreeNode buildTree(int[] preorder, int[] inorder) {
        
                    if (preorder == null || inorder == null) {
        
                return null;
            }
            return creatTreeByPandI(preorder,inorder,0,inorder.length-1);
        }
    }

![202303282044806.png][]

## ?6.根据一棵树的中序遍历与后序遍历构造二叉树? ##

根据一棵树的中序遍历与后序遍历构造二叉树[OJ链接][OJ 5]

![202303282044116.png][] ![202303282044430.png][] ![202303282044572.png][]

思路：

1、先将pi下标的 元素 创建为root

2、在中序遍历的数组当中，找到当前pi下标的元素，存在的位置。ri

3、先找根，然后在找右子树，最后找左子树。

/**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode() {}
     *     TreeNode(int val) { this.val = val; }
     *     TreeNode(int val, TreeNode left, TreeNode right) {
     *         this.val = val;
     *         this.left = left;
     *         this.right = right;
     *     }
     * }
     */
    class Solution {
        
        public int postIndex = 0;
        public TreeNode creatTreeByPandI(int[] inorder,int[] postorder, int inbegin, int inend) {
        
            if(inbegin > inend) {
        
                //如果满足这个条件 说明 没有左树 或者 右树了
                return null;
            }
            TreeNode root = new TreeNode(postorder[postIndex]);
            //找到根在中序遍历的位置
            int rootIndex = findIndexOfI(inorder,inbegin,inend,postorder[postIndex]);
            if (rootIndex == -1) {
        
                return null;
            }
            postIndex--;
            //分别找到右子树和左子树
            root.right = creatTreeByPandI(inorder,postorder,rootIndex + 1,inend);
            root.left = creatTreeByPandI(inorder,postorder,inbegin,rootIndex - 1);
            return  root;
        }
        private int findIndexOfI(int[] inorder, int inbegin, int inend,int key) {
        
            for (int i = inbegin; i <= inend; i++) {
        
                if (inorder[i] == key) {
        
                    return i;
                }
            }
            return -1;
        }
        public TreeNode buildTree(int[] inorder, int[] postorder) {
        
            if (postorder == null || inorder == null) {
        
                return null;
            }
            postIndex = postorder.length-1;
    
            return creatTreeByPandI(inorder,postorder,0,inorder.length-1);
        }
    }

![202303282045980.png][]

## ?7.二叉树创建字符串? ##

二叉树创建字符串[OJ链接][OJ 6]

![202303282045978.png][] ![202303282045836.png][]

/**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode() {}
     *     TreeNode(int val) { this.val = val; }
     *     TreeNode(int val, TreeNode left, TreeNode right) {
     *         this.val = val;
     *         this.left = left;
     *         this.right = right;
     *     }
     * }
     */
    class Solution {
        
        public void treeToString(TreeNode t,StringBuilder sb) {
        
            if (t == null) {
        
                return;
            }
            sb.append(t.val);
            if (t.left != null) {
        
                sb.append("(");
                treeToString(t.left,sb);
                sb.append(")");
            } else {
        
                //t.left == null;
                if (t.right == null) {
        
                    return;
                } else {
        
                    sb.append("()");
                }
            }
            if (t.right == null) {
        
                return;
            } else {
        
                sb.append("(");
                treeToString(t.right,sb);
                sb.append(")");
            }
            
        }
        public String tree2str(TreeNode root) {
        
            if (root == null) {
        
                return null;
            }
            StringBuilder sb = new StringBuilder();
            treeToString(root,sb);
            return sb.toString();
        }
    }

![202303282045407.png][]

思路：例如上图，我们一直往左，每添加一个元素就加一个"(“，再往右判断如果右也为空我们就添加一个”)“，然后返回到上一个元素，如果右边有元素也是重复之前的操作，添加一个”(“然后继续往后判断，左右为空就添加一个”)“，如果左树为空右树不为空，我们就添加一个”()"。

![202303282046301.png][] ![202303282047200.png][]

*感谢各位读者的阅读，本文章有任何错误都可以在评论区发表你们的意见，我会对文章进行改正的。如果本文章对你有帮助请动一动你们敏捷的小手点一点赞，你的每一次鼓励都是作者创作的动力哦！?*

[OJ]: https://www.nowcoder.com/practice/4b91205483694f449f94c179883c1fef?tpId=60&&tqId=29483&rp=1&ru=/activity/oj&qru=/ta/tsing-kaoyan/question-ranking
[202303282037607.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282037607.png
[202303282038282.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/be450817f591421f8eb8a2b99932029e.png
[202303282038978.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/733c45dd02464b858de2cac8eb8104a7.png
[202303282038091.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/73be699dab3a4241b6038d3415ea5433.png
[OJ 1]: https://leetcode.cn/problems/binary-tree-level-order-traversal/
[202303282039347.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282039347.png
[202303282039394.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/ac61bf6c61094121b92664f7298049ad.png
[202303282039762.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/3cbd6d40e3454a06a1b7166cb1babb2c.png
[OJ 2]: https://leetcode.cn/problems/lowest-common-ancestor-of-a-binary-tree/
[202303282039800.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282039800.png
[202303282039811.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282039811.png
[202303282039143.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/40df3114f2b346849bf50546333bb9e9.png
[202303282040515.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/6aaa441b876d4a3d988538e6d7fb0d92.png
[202303282040214.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/07396ac4647d4d6cb61b8c70fffe3741.png
[202303282040330.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/7725e5636d6e407b84b0f87fa402596a.png
[202303282041110.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/d1fb6be383534179a56ef707c8b40025.png
[202303282041429.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282041429.png
[202303282047890.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/b074e37163d94a6898f18ba872c88e0e.png
[OJ 3]: https://www.nowcoder.com/practice/947f6eb80d944a84850b0538bf0ec3a5?tpId=13&&tqId=11179&rp=1&ru=/activity/oj&qru=/ta/coding-interviews/question-ranking
[202303282042759.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282042759.png
[202303282042152.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282042152.png
[202303282042971.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/290db4b9dbd5400388ba358a04cffff3.png
[202303282043745.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/e87d4181e60642b9bab403986c665336.png
[OJ 4]: https://leetcode.cn/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
[202303282043930.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282043930.png
[202303282043657.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282043657.png
[202303282043833.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282043833.png
[202303282044198.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/6d85852e32764d8ca3b07e3b28078f55.png
[202303282044806.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/63fa527f8d854ac9a016eef3815e4b6b.png
[OJ 5]: https://leetcode.cn/problems/construct-binary-tree-from-inorder-and-postorder-traversal/
[202303282044116.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282044116.png
[202303282044430.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282044430.png
[202303282044572.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/b992551ae8f04f76b14d0db6078cabc5.png
[202303282045980.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/e43665d2877e4e9fa0068c31b4e7d56a.png
[OJ 6]: https://leetcode.cn/problems/construct-string-from-binary-tree/
[202303282045978.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282045978.png
[202303282045836.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/680c17193e26462daeb4975ac769daca.png
[202303282045407.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/a5bad8397cd64f068c47ed3b0d5636a4.png
[202303282046301.png]: https://7078909xxh.oss-cn-shanghai.aliyuncs.com/csdn图床专用区/202303282046301.png
[202303282047200.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/28/4f1c9544f57b4761a8ce7007c4300215.png