[Tree]Binary Tree Preorder Traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        TreeNode* p = root;
        if(p==NULL){
            return res;
        }
        stack<TreeNode*> stk;
        while(p || !stk.empty()){
            while(p){
                res.push_back(p->val);
                stk.push(p);
                p=p->left;
            }
            if(!stk.empty()){
                p = stk.top()->right;
                stk.pop();
            }
        }
        return res;
    }
};