leetcode.226 反转二叉树

题目描述：给一个二叉树，反转它

Invert a binary tree.

Example:

Input:

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

Output:

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

Trivia:
This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so f*** off.

思路：

递归，从根节点出发，反转当前节点的左子树和右子树

边界条件：当前节点没有子节点就返回，否则继续递归

递归前进段：当前节点有子节点

递归返回段：当前节点没有子节点

代码实现：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        // 思路
        // 递归来做，从根节点出发，将其左子树和右子树位置进行调换
        // 边界条件，当前节点没有子节点了，那么就返回，否则继续递归
        // 递归前进段：当前节点有子节点
        // 递归返回段：当前节点没有子节点
        if(root == null)
            return null;
        TreeNode tmp = root.left;
        root.left = invertTree(root.right);
        root.right = invertTree(tmp);
        return root;
    }
}

leetcode.226 反转二叉树

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