https://leetcode.com/problems/invert-binary-tree/

1. Use recursion:

  1. //4 ms    8.5 MB
  3. /**
  4.  * Definition for a binary tree node.
  5.  * struct TreeNode {
  6.  *     int val;
  7.  *     TreeNode *left;
  8.  *     TreeNode *right;
  9.  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
  10.  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
  11.  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
  12.  * };
  13.  */
  14. class Solution {
  15. public:
  16.     TreeNode* invertTree(TreeNode* root) {
  17.         if(!root) return NULL;
  19.         TreeNode* left = invertTree(root->left);
  20.         TreeNode* right = invertTree(root->right);
  21.         root->left = right;
  22.         root->right = left;
  24.         return root;
  25.     }
  26. };

Time complexity: O(n)
Space complexity: O(n)