leetcode 116: Populating Next Right Pointers in Each Node

/** * Definition for binary tree with next pointer. * struct TreeLinkNode { *  int val; *  TreeLinkNode *left, *right, *next; *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {} * }; */class Solution {public:    void connect(TreeLinkNode *root) {        vector<TreeLinkNode*> last;        helper(root,0,last);    }    void helper(TreeLinkNode *root,int level,vector<TreeLinkNode*>& last)    {        if(!root)            return;        if(last.size()<=level)            last.push_back(root);        else        {            last[level]->next=root;            last[level]=root;        }        helper(root->left,level+1,last);        helper(root->right,level+1,last);    }};