二叉树的前、中、后序遍历和层序遍历的实现（递归 + 迭代）-蒲公英云

二叉树的前、中、后序遍历和层序遍历的实现（递归 + 迭代）

女爷i 2023-09-28 23:17 19阅读 0赞

该篇文章写了二叉树的四种遍历方式，分别对应力扣上的题目为144. 二叉树的前序遍历、94. 二叉树的中序遍历、145. 二叉树的后序遍历、102. 二叉树的层序遍历。

二叉树结点定义：

public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        public TreeNode() {
        }
        public TreeNode(int val) {
            this.val = val;
        }
        public TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }

二叉树的递归遍历：

前序遍历：

public void Preorder(List<Integer> list, TreeNode root) {
        if (root == null) {
            return;
        }
        //中-左-右
        list.add(root.val);
        Preorder(list, root.left);
        Preorder(list, root.right);
    }

中序遍历：

public void MesoOrder(List<Integer> list, TreeNode root) {
        if (root == null) {
            return;
        }
        //左-中-右
        MesoOrder(list, root.left);
        list.add(root.val);
        MesoOrder(list, root.right);
    }

后序遍历：

public void Postorder(List<Integer> list, TreeNode root) {
        if (root == null) {
            return;
        }
        //左-右-中
        Postorder(list, root.left);
        Postorder(list, root.right);
        list.add(root.val);
    }

层序遍历：

public void Sequence(List<List<Integer>> list, TreeNode root, int deep) {
        if (root == null) {
            return;
        }
        if (list.size() == deep) {
            list.add(new ArrayList<>());
        }
        list.get(deep).add(root.val);
        Sequence(list, root.left, deep + 1);
        Sequence(list, root.right, deep + 1);
    }

二叉树的迭代遍历：

前序遍历：

public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        if (root != null) {
            stack.push(root);
        }
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            res.add(cur.val);
            if (cur.right != null) {
                stack.push(cur.right);
            }
            if (cur.left != null) {
                stack.push(cur.left);
            }
        }
        return res;
    }

中序遍历：

public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()) {
            if (cur != null) {
                stack.push(cur);
                cur = cur.left;
            } else {
                cur = stack.pop();
                res.add(cur.val);
                cur = cur.right;
            }
        }
        return res;
    }

后续遍历：

public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        if (root != null) {
            stack.push(root);
        }
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            res.add(cur.val);
            if (cur.left != null) {
                stack.push(cur.left);
            }
            if (cur.right != null) {
                stack.push(cur.right);
            }
        }
        Collections.reverse(res);
        return res;
    }

层序遍历：

public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        if (root != null) {
            stack.offer(root);
        }
        while (!stack.isEmpty()) {
            int size = stack.size();
            List<Integer> list = new ArrayList<>();
            while (size-- != 0) {
                TreeNode node = stack.poll();
                list.add(node.val);
                if (node.left != null) {
                    stack.offer(node.left);
                }
                if (node.right != null) {
                    stack.offer(node.right);
                }
            }
            res.add(list);
        }
        return res;
    }