108. Convert Sorted Array to Binary Search Tree

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

【思路】中间节点作为根节点，左边序列的中间节点作为左子树的根节点，右边序列的中间节点作为右子树的根节点。

/** * 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* dfs(vector<int>& nums, int first, int last){        if(first <= last)        {            int mid =  first +(last -first ) / 2;             TreeNode* root = new TreeNode(nums[mid]);            root->left = dfs(nums, first, mid-1);            root->right = dfs(nums,mid+1, last);            return root;        }        else        return NULL; }    TreeNode* sortedArrayToBST(vector<int>& nums) {       if(nums.size()==0) return NULL;       TreeNode* root = dfs(nums, 0, nums.size()-1);       return root;    }};