226. Invert Binary Tree

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) {
        invert(root);
        return root;
    }

    void invert(TreeNode root){
        if(root == null) return;
        TreeNode l = root.left;
        root.left = root.right;
        root.right = l;
        invert(root.left);
        invert(root.right);
    }
}


//solution 2:
public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null) return root;
        TreeNode invertLeft = invertTree(root.left);
        root.left = invertTree(root.right);
        root.right = invertLeft;
        return root;
    }
}