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.

解题思路：升序数组构建而查找树，通过109的讲解（详情可见），我们发现最优解是中序遍历构建是最优解，而与升序链表不同的地方在于它的size是直接获取的；

class Solution {
private:
int count = 0;
vector<int> ls;
TreeNode* helpsortedArrayToBST(int size){
if (size == 0) return NULL;
TreeNode* root = new TreeNode(0);
root->left = helpsortedArrayToBST(size/2);
root->val = ls[count++];
root->right = helpsortedArrayToBST(size-size/2- 1);
return root;
}
public:
TreeNode* sortedArrayToBST(vector<int>& nums) {
ls.assign(nums.begin(),nums.end());
return helpsortedArrayToBST(nums.size());
}
};结果如下：