Binary Tree Preorder Traversal.cpp

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> nodes;
        stack<TreeNode*> todo;
        while (root || !todo.empty()) {
            if (root) {
                nodes.push_back(root -> val);
                if (root -> right) {
                    todo.push(root -> right);
                }
                root = root -> left;
            } else {
                root = todo.top();
                todo.pop();
            }
        }
        return nodes;
    }
};