【LeetCode】Convert Sorted Array to Binary Search Tree ---递归建立二叉搜索树

description：

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

code：Hint:类似于二分搜索，找到中点值作为根，再递归地建立左右子树

/**
 * 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<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(num.size() == 0)
        {
            return NULL;
        }
        return sorted2bst(num,0,num.size()-1);
        
    }
    TreeNode *sorted2bst(vector<int> &num, int st, int end)
    {
        if(st<=end)
        {
            int mid = (st+end)>>1;
            TreeNode *root = new TreeNode(num[mid]);
            root->left = sorted2bst(num,st,mid-1);
            root->right = sorted2bst(num,mid+1,end);
            return root;
        }
        else return NULL;
    }
};