LeetCode 144. Binary Tree Preorder Traversal

// Given a binary tree, return the preOrder traversal of its nodes' value
// The main idea here is to use stack, nothing special

vector<int> preorderTraversal(TreeNode* root) {
    vector<int> result;
    if(!root) return result;
    stack<TreeNode*> nodes;
    nodes.push(root);
    while(!nodes.empty()) {
        TreeNode* tmp = nodes.top();
        nodes.pop();                     // remember to pop it out.
        result.push_back(tmp->val);
        if(tmp->right != NULL)
            nodes.push(tmp->right);
        if(tmp->left != NULL)
            nodes.push(tmp->left);
    }
}

Preorder iterator

class PreorderIterator {
private:
  stack<TreeNode*> nodes;

public:
  void preorderIterator(TreeNode* root) {
    nodes.push_back(root);
  }
  void hasNext() {
    return !nodes.empty();
  }

  void getNext() {
    if(hasNext()) {
      TreeNode* top = nodes.top();
      if(top->right) nodes.push_back(top->right);
      if(top->left) nodes.push_back(top->left);
      nodes.pop();
    }
  }
}