226_InvertBinaryTree.java

// 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.



/**
 * 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) {
        helper(root);
        return root;
    }
    public void helper(TreeNode root) {
        if(root == null) return;
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        helper(root.left);
        helper(root.right);
    }
}