leetcode 226： Invert Binary Tree

题目：

Invert a binary tree.

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

to

     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 fuck off.

 

思路：

置换，用递归，左右子树置换

代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null) 
            return root;
        else {
            if(root.left!=null && root.right!=null) {
                TreeNode temp;
                temp = root.left;
                root.left = root.right;
                root.right = temp;
                
                invertTree(root.left);
                invertTree(root.right);
            } else if(root.left == null && root.right !=null) {
                root.left = root.right;
                root.right = null;
                invertTree(root.left);
            } else if(root.left != null && root.right ==null) {
                root.right = root.left;
                root.left = null;
                invertTree(root.right);
            } else {
                return root;
            }
        }
        return root;
    }
}