Invert Binary Tree

Invert Binary Tree

Question:

Given the root of a binary tree, invert the tree (against the y-axis), and return its root.

How to Solve:

We create a helper function swapLR(root) that takes in one TreeNode argument and swaps the left subtree and right subtree of that argument. It's a recursive function that calls swapLR(root->left) and swapLR(root->right), before actually swapping its left and right children using the classic "tmp=a; a=b; b=tmp" technique. Because we should start swapping the subtrees from bottom up.

The base case is when nullptr is hit as root.

My C++ Solution:

class Solution {
 public:
  auto invertTree(TreeNode *root) -> TreeNode * {
    swapLR(root);
    return root;
  }

 private:
  void swapLR(TreeNode *root) {
    if (!root) return;

    swapLR(root->left);
    swapLR(root->right);

    auto *tmp = root->left;
    root->left = root->right;
    root->right = tmp;
  }
};