leetcode 108. Convert Sorted Array to Binary Search Tree

题目描述：

//用一个升序的数组 构建 高度平衡的儿茶搜索树

//用递归和二分的思想 

/**
 * 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:
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        return dfs(nums,0,nums.size()-1);
    }
    TreeNode *dfs(vector<int> &nums, int start, int end){
        if(start > end) return NULL;
        int mid = start + (end - start)/2;
        TreeNode *node = new TreeNode(nums[mid]);
        node->left = dfs(nums,start,mid-1);
        node->right = dfs(nums,mid+1,end);
        
        return node;
    }
};