108. Convert Sorted Array to Binary Search Tree

https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/

1. Use BST:

//24 ms    23.5 MB
/**
 * 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* sortedArrayToBST(vector<int>& nums) {
        //input is already sorted
        int l = 0, r = nums.size() - 1;
        TreeNode* root = build(nums, l, r);
        return root;
    }
private: 
    TreeNode* build(vector<int>& nums, int l, int r) {
        if(l > r) return NULL;
        int median = l + (r - l) / 2;
        TreeNode* root = new TreeNode(nums[median]);
        root->left = build(nums, l, median - 1);
        root->right = build(nums, median + 1, r);
        return root;
    }
};