【剑指offer第六天】搜索与回溯算法

待我称王封你为后i 2022-09-04 05:50 108阅读 0赞

## 第一题 层次遍历二叉树 ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgxMjYwOQ_size_16_color_FFFFFF_t_70]  
使用队列实现  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgxMjYwOQ_size_16_color_FFFFFF_t_70 1]

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */
    class Solution { 
        public int[] levelOrder(TreeNode root) { 
            //层次遍历
            if(root == null){ return new int[0];}
            Deque<TreeNode> queue = new LinkedList<>();
            queue.add(root);
            ArrayList<Integer> tmp = new ArrayList<>();
            while(!queue.isEmpty()){ 
                TreeNode node = queue.poll();
                tmp.add(node.val);
                if(node.left != null){ queue.add(node.left);}
                if(node.right != null){ queue.add(node.right);}
            }
            int []res = new int[tmp.size()];
            for(int i = 0; i < tmp.size(); i++){ 
                res[i] = tmp.get(i);
            }
            return res;
        }
    }

### 第二题 进行层次遍历并按层输出 ###

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgxMjYwOQ_size_16_color_FFFFFF_t_70 2]

java中Queue类只能使用LinkedList创建

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */
    class Solution { 
        public List<List<Integer>> levelOrder(TreeNode root) { 
            List<List<Integer>> res = new ArrayList<>();
            Queue<TreeNode> queue = new LinkedList<>();
            if(root != null){ queue.add(root);}
            while(!queue.isEmpty()){ 
                List<Integer> tmp = new ArrayList<>();
                //这里遍历时 要从queue.size()开始倒序遍历，因为在下面的循环中queue.size()会随着变化
                for(int i = queue.size();i > 0;i--){ 
                    TreeNode node = queue.poll();
                    tmp.add(node.val);
                    if(node.left!=null){ queue.add(node.left);}
                    if(node.right!=null){ queue.add(node.right);}
                }
                res.add(tmp);
            }
            return res;
        }
    }

### 第三题 层次遍历 ###

之字型打印遍历结果，按照奇偶分别在头尾插入到队列中  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgxMjYwOQ_size_16_color_FFFFFF_t_70 3]

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */
    class Solution { 
        public List<List<Integer>> levelOrder(TreeNode root) { 
            List<List<Integer>> res = new ArrayList<>();
            Queue<TreeNode> queue = new LinkedList<>();
            if(root != null){ queue.add(root);}
            while(!queue.isEmpty()){ 
                LinkedList<Integer> tmp = new LinkedList<>();
                for(int i = queue.size();i > 0;i--){ 
                    TreeNode node = queue.poll();
                    if(res.size()%2==0){ 
                        tmp.addLast(node.val);
                    }else{ 
                        tmp.addFirst(node.val);
                    }
                    if(node.left!=null){ queue.add(node.left);}
                    if(node.right!=null){ queue.add(node.right);}
                }
                res.add(tmp);
            } 
            return res;
        }
    }

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