Invert the Binary Tree

Given a binary tree, invert the binary tree and return it.
Look at the example for more details.

Example :
Given binary tree

     1
   /   \
  2     3
 / \   / \
4   5 6   7

invert and return

     1
   /   \
  3     2
 / \   / \
7   6 5   4

Method:

DFS

Solution:

Time: O(n)
Space: O(1)

/**
 * Definition for binary tree
 * class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) {
 *      val = x;
 *      left=null;
 *      right=null;
 *     }
 * }
 */
public class Solution {
    public TreeNode invertTree(TreeNode A) {
        if (A == null) return A;
        if (A.left == null && A.right == null) {
            return A;
        }
        TreeNode left = A.left;
        TreeNode right = A.right;
        A.right = invertTree(left);
        A.left = invertTree(right);
        return A;
    }
}