[Leetcode 226, Easy] Invert Binary Tree

Problem:

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.

Analysis:

Solutions:

C++:

    TreeNode* invertTree(TreeNode* root) {
        if (root == NULL)
            return root;
        TreeNode* p_left_subtree_root = invertTree(root->left);
        TreeNode* p_right_subtree_root = invertTree(root->right);
        root->left = p_right_subtree_root;
        root->right = p_left_subtree_root;
        return root;
    }

Java :

Python: