Iterative Preorder Traversal

We need to use a stack for a iterative traversal and traverse from right to left , as stack follows ( Last In First Out)

Below is the implementation of the code:-

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;
    }
};

Time Complexity :O(n) , because we are travelling every node
Space Complexity :O(n) , ( this is the worst case in case of
skewed tree)