1. preorder travel

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

2. inorder travel

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

3. postorder travel

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> result;
        stack<TreeNode *> myStack;
        TreeNode * temp = root;
        while(!myStack.empty() || temp != NULL){
            if(temp != NULL){
                myStack.push(temp);
                result.insert(result.begin(),temp->val);
                temp = temp->right;
            }
            else{
                TreeNode * node = myStack.top();
                myStack.pop();
                temp = node->left;
            }
        }
        return result;
    }
};