#Leetcode# 144. Binary Tree Preorder Traversal

https://leetcode.com/problems/binary-tree-preorder-traversal/

 

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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

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

代码：

/**
 * 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> ans;
        if(!root) return ans;
        pre(ans, root);
        return ans;
    }
    void pre(vector<int>& ans, TreeNode* root) {
            ans.push_back(root -> val);
            if(root -> left)
                pre(ans, root -> left);
            if(root -> right)
                pre(ans, root -> right);
    }
};

　　今天刷完了《妻子的浪漫旅行》 可以安心学习啦