leetcode - [7]Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].

思路：前序遍历，“根，左子树，右子树”

 #include <iostream>
 #include <vector>
 #include <stack>
 using namespace std;

 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;
         stack<TreeNode*> s;
         if (!root) {
             return res;
         }

         s.push(root);
         while(!s.empty()) {
             TreeNode *p = s.top(); s.pop();
             res.push_back(p->val);
             if (p->right) {
                 s.push(p->right);
             }
             if (p->left) {
                 s.push(p->left);
             }
         }
         return res;
     }
 };

 int main() {
     return ;
 }