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;
}
};