【LeetCode】108. Convert Sorted Array to Binary Search Tree

Convert Sorted Array to Binary Search Tree

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

这题思路参照Convert Sorted List to Binary Search Tree，由于vector可以用下标访问，因此不用设置快慢节点，直接访问中间点。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode *sortedArrayToBST(vector<int> &num) {
        return Helper(num, 0, num.size()-1); 
    }
    TreeNode* Helper(vector<int>& num, int begin, int end)
    {
        if(begin > end)
            return NULL;
        else if(begin == end)
            return new TreeNode(num[begin]);
        
        int mid = begin + (end-begin)/2;
        TreeNode* root = new TreeNode(num[mid]);
        root->left = Helper(num, begin, mid-1);
        root->right = Helper(num, mid+1, end);
        return root;
    }
};