二叉树----3.翻转二叉树

题目链接

递归终止条件：当前节点为空时返回null

后序遍历：先递归翻转左子树和右子树

回溯时交换当前节点的左右子树

/**
 * 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 TreeNode invertTree(TreeNode root) {
        // 递归终止条件：当前节点为空时返回null
        // 后序遍历：先递归翻转左子树和右子树
        // 回溯时交换当前节点的左右子树

        if(root == null) {
            return null;
        }

        TreeNode leftRoot = invertTree(root.left);
        TreeNode rightRoot = invertTree(root.right);

        //交换
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;

        return root;
    }

}