LeetCode -226.翻转二叉树 -Java

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/invert-binary-tree

翻转一棵二叉树。

示例：

输入：

     4
   /   \
  2     7
 / \   / \
1   3 6   9

输出：

     4
   /   \
  7     2
 / \   / \
9   6 3   1

解题思路：
翻转整棵树就是交换每个节点的左右子节点

/**
 * 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) {
    
    
        if(root == null) {
    
    
            return null;
        }
		//交换root节点的左右子节点
        TreeNode temp;
        temp = root.left;
        root.left = root.right;
        root.right = temp;
		//同样，左右子节点的左右子节点也继续翻转
        invertTree(root.left);
        invertTree(root.right);
        
        return root;
    }
}