LeetCode: Binary Tree Preorder Traversal

vector<int> preorderTraversal(TreeNode *root) {vector<int> res;if (!root){return res;}stack<TreeNode*> nodes;nodes.push(root);while (!nodes.empty()){TreeNode* cur = nodes.top();nodes.pop();res.push_back(cur->val);if (cur->right){nodes.push(cur->right);}if (cur->left){nodes.push(cur->left);}}return res;}