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.

Solution

/**
 * 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) {
        if(root==null) return root;;
        TreeNode tmp=root.right;
        root.right=root.left;
        root.left=tmp;
        invertTree(root.left);
        invertTree(root.right);
        return root;
    }
}

时间O(n)
空间O(h)

和二叉树的镜像有一拼，不过这个更简单，就是从上往下换左右节点，没了