leetcode 144 二叉树的前序遍历

地址：https://leetcode-cn.com/problems/binary-tree-preorder-traversal/submissions/
大意：前序遍历一棵树

/**
 * 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) {
        stack<TreeNode*> stk;
        stk.push(root);
        vector<int> res;

        while(stk.size() != 0){
            TreeNode *tre = stk.top();
            stk.pop();
            if(tre == NULL)
                continue;
            res.push_back(tre->val);
            stk.push(tre->right);
            stk.push(tre->left);
        }
        return res;
    }
};