108. Convert Sorted Array to Binary Search Tree（从有序数组中构造平衡的BST）

题意：从有序数组中构造平衡的BST。

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