226. 翻转二叉树

226. 翻转二叉树

已解答

简单

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给你一棵二叉树的根节点 root ，翻转这棵二叉树，并返回其根节点。

 

示例 1：

输入： root = [4,2,7,1,3,6,9]
输出： [4,7,2,9,6,3,1]

示例 2：

输入： root = [2,1,3]
输出： [2,3,1]

示例 3：

输入： root = []
输出： []

 

提示：

• 树中节点数目范围在 [0, 100] 内
• -100 <= Node.val <= 100

解题答案

/**
 * 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;
 *     }
 * }
 */
// 栈
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) return root;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            TreeNode left = node.left;
            node.left = node.right;
            node.right = left;
            if (node.left != null) {
                stack.push(node.left);
            }
            if (node.right != null) {
                stack.push(node.right);
            }
        }
        return root;
    }
}

/**
 * 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;
 *     }
 * }
 */
// 自上向下迭代
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) return root;
        TreeNode left = root.left;
        root.left = root.right;
        root.right = left;
        invertTree(root.left);
        invertTree(root.right);
        return root;
    }
}

/**
 * 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;
 *     }
 * }
 */
// 自底向上递归
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        TreeNode left = invertTree(root.left);
        TreeNode right = invertTree(root.right);
        root.left = right;
        root.right = left;
        return root;
    }
}