*(leetcode) Convert Sorted Array to Binary Search Tree (tree)

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

递归构造BST，没什么难度。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
    TreeNode *bst(vector<int> &num, int left, int right){
        if(left>right)
            return NULL;
        if(left==right){
            TreeNode *node = new TreeNode(num[left]);
            return node;
        }
        int mid = (left+right)/2;
        TreeNode *leftBst = bst(num,left, mid-1);
        TreeNode *rightBst = bst(num, mid+1, right);
        TreeNode *root = new TreeNode(num[mid]);
        root->left = leftBst;
        root->right = rightBst;
        return root;
    }
public:
    TreeNode *sortedArrayToBST(vector<int> &num) {
        return bst(num, 0, num.size()-1);
    }
};