226. Invert Binary Tree

https://leetcode.com/problems/invert-binary-tree/description/
解题思路：用preoder traversal解决

代码：
class Solution {
public TreeNode invertTree(TreeNode root) {
if(root == null) return null;

    TreeNode temp = root.left;
    root.left = invertTree(root.right);
    root.right = invertTree(temp);
    return root;
}

}