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.

分析：对于BST，左右子树的高度差小于等于1，则在由sorted array构建BST时把数组中间元素作为根节点，然后递归的构建左右子树。代码如下：

class Solution {
public:
    TreeNode *sortedArrayToBST(vector<int> &num) {
        return build_BST(num, 0, num.size()-1);
    }
    
    TreeNode *build_BST(vector<int> &num, int start, int end){
        if(start > end) return NULL;
        
        int mid = (start + end)/2;
        TreeNode *root = new TreeNode(num[mid]);
        root->left = build_BST(num, start, mid-1);
        root->right = build_BST(num, mid+1, end);
        
        return root;
    }
};