main.cpp

// Source: https://leetcode.com/problems/invert-binary-tree
// Title: Invert Binary Tree
// Difficulty: Easy
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Given the `root` of a binary tree, invert the tree, and return its root.
//
// **Example 1:**
// https://assets.leetcode.com/uploads/2021/03/14/invert1-tree.jpg
//
// ```
// Input: root = [4,2,7,1,3,6,9]
// Output: [4,7,2,9,6,3,1]
// ```
//
// **Example 2:**
// https://assets.leetcode.com/uploads/2021/03/14/invert2-tree.jpg
//
// ```
// Input: root = [2,1,3]
// Output: [2,3,1]
// ```
//
// **Example 3:**
//
// ```
// Input: root = []
// Output: []
// ```
//
// **Constraints:**
//
// - The number of nodes in the tree is in the range `[0, 100]`.
// - `-100 <= Node.val <= 100`
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include <utility>
using namespace std;

struct TreeNode {
  int val;
  TreeNode *left;
  TreeNode *right;
  TreeNode() : val(0), left(nullptr), right(nullptr) {}
  TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
  TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

class Solution {
 public:
  TreeNode *invertTree(TreeNode *root) {
    if (!root) return nullptr;

    swap(root->left, root->right);
    invertTree(root->left);
    invertTree(root->right);

    return root;
  }
};