Iterative Preorder Traversal

 class Solution { public: vector<int> preorderTraversal(TreeNode* root) { //we need to declare a vector vector<int> preorder; //if the tree is empty , you return empty preorder if(root == NULL) return preorder; //you then initialize the stack stack<TreeNode*>st; st.push(root); //you keep iterating on the stack till it is non-empty while(!st.empty()){ //now you get the top most element root = st.top(); st.pop(); preorder.push_back(root->val); //check if it has a right or left on it and put it into the stack if(root->right!=NULL){ st.push(root->right); } if(root->left!=NULL){ st.push(root->left); } } return preorder; } };