LeetCode 226: Invert Binary Tree

/**
 * Definition for a binary tree node.
 * 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 == NULL) return NULL;

		TreeNode * left = root->left;
		TreeNode * right = root->right;
		root->left = invertTree(right);
		root->right = invertTree(left);

		return root;
	}
};