[LeetCode]Convert Sorted Array to Binary Search Tree

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

平分，递归。

/**
 * 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 *help(vector<int> &num,int a,int b){
		int len  = b - a + 1;
		if(len <= 0 )
			return NULL;
		int mid  = a+len/2;
		TreeNode *p = new TreeNode(num[mid]);
		if(len ==1)
			return p;
		int left_len = mid - a;
		int right_len = b - mid;
		if(left_len > 0)
			p->left = help(num,a,a+left_len-1);
		if(right_len > 0)
			p->right = help(num,a+left_len+1, b);
		return p;
	}
    TreeNode *sortedArrayToBST(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int len = num.size();
		if(len==0)
			return NULL;
		return help(num,0,len-1);
		
    }
};