Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

 

这题思路参照，由于vector可以用下标访问，因此不用设置快慢节点，直接访问中间点。

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    TreeNode *sortedArrayToBST(vector
     
       &num) {        return Helper(num, 0, num.size()-1);     }    TreeNode* Helper(vector
      
       & num, int begin, int end)    {        if(begin > end)            return NULL;        else if(begin == end)            return new TreeNode(num[begin]);                int mid = begin + (end-begin)/2;        TreeNode* root = new TreeNode(num[mid]);        root->left = Helper(num, begin, mid-1);        root->right = Helper(num, mid+1, end);        return root;    }};