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].

此题之前和文清讨论过，一次过了。但是总体难度上，比后续迭代简单不少。

/**
 * Definition for binary tree
 * 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> re;
        if(root == NULL)return re;
        stack<TreeNode*> sk;
        sk.push(root);
        while(!sk.empty())
        {
            TreeNode *now = sk.top();
            sk.pop();
            re.push_back(now->val);
            if(now->right != NULL)
            sk.push(now->right);
            if(now->left != NULL)
            sk.push(now->left);
            
        }
        return re;
    }
};