题目链接：https://oj.leetcode.com/problems/binary-tree-preorder-traversal/

题目：

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].

Note: Recursive solution is trivial, could you do it iteratively?

解题思路：比较基础的树的前序遍历

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    vector<int> s;
public:
    vector<int> preorderTraversal(TreeNode *root) {
        preOrder(root);
        return s;
    }
    void preOrder(TreeNode *root) {
        if (root == NULL) return;
        s.push_back(root->val);
        preOrder(root->left);
        preOrder(root->right);
    }
};

Leetcode-Binary Tree Preorder Traversal