108. Convert Sorted Array to Binary Search Tree.cpp

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

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted array: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5
*/
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        if ( nums.empty() ) return nullptr;
        return convert( nums, 0, nums.size() - 1 );
    }
    TreeNode* convert( vector<int>& nums, int begin, int end )
    {
        if ( begin > end ) return nullptr;
        int mid = begin + ( end - begin ) / 2;
        TreeNode *root = new TreeNode( nums[mid] );
        root->left = convert( nums, begin, mid - 1 );
        root->right = convert( nums, mid + 1, end );
        return root;
    }
};