[Leetcode] Convert Sorted Array to Binary Search Tree

题目：

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

思路：类似二分法，中间向左右两边找child.

class Solution {
public:
    TreeNode* buildBST_helper(const vector<int>& num, int lower_bound, int upper_bound) {
        if (lower_bound > upper_bound) return nullptr;
        int mid = (lower_bound + upper_bound) / 2;
        TreeNode* new_root = new TreeNode(num[mid]);
        new_root->left = buildBST_helper(num, lower_bound, mid - 1);
        new_root->right = buildBST_helper(num, mid + 1, upper_bound);
        return new_root;
    }
    
    TreeNode *sortedArrayToBST(vector<int> &num) {
        return buildBST_helper(num, 0, num.size() - 1);
    }
};

总结：复杂度为O(n).