【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;
}
};