[leetcode] Tree Traversal

1. preorder traversal
Previously I did the preorder traversal the same way as inorder traversal, however, there is a more simplified way to do that.

vector<int> preorderTraversal(TreeNode *root) {
        vector<int> ret;
        if(!root) return ret;
        stack s;
        s.push(root);
        while(!s.empty()){
            TreeNode* cur = s.top(); s.pop();
            ret.push_back(cur->val);
            if(cur->right) s.push(cur->right);
            if(cur->left) s.push(cur->left);
        }
        return ret;
    }