LeetCode OJ - Convert Sorted Array to Binary Search Tree

题目：

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

解题思路：

同链表的方法一样。一分为二，递归进行求解。

代码：

/**
 * 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* BuildTree(vector<int> &num, int start, int end) {
        if (start == end) {
            return NULL;
        }
        int middle = start + (end - start) / 2;
        TreeNode* root = new TreeNode(num[middle]);
        TreeNode* left_child = BuildTree(num, start, middle);
        TreeNode* right_child = BuildTree(num, middle + 1, end);
        root->left = left_child;
        root->right = right_child;
        return root;
    }
    TreeNode *sortedArrayToBST(vector<int> &num) {
        return BuildTree(num, 0, num.size());
    }
};