226. Invert Binary Tree

Invert a binary tree.

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

to

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

//用一个temp交换左右tree再recursion就可以了

public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null){
            return null;
        }
        TreeNode temp = root.right;
        root.right = root.left;
        root.left = temp;
        
        invertTree(root.left);
        invertTree(root.right);
        return root;
    }
}