226. Invert Binary Tree

Invert a binary tree.

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

to

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

思路：递归。用invert左子树，存在一个node，invert右子树，存在另一个node。然后左右互换即可。今天学校食堂居然没来熊猫快递，啃了个破汉堡＝ ＝。

/**
 * 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;
        }
        
        TreeNode l1=invertTree(root.left);
        TreeNode l2=invertTree(root.right);
        root.right=l1;
        root.left=l2;
        return root;
        
    }
}