26. Convert Sorted Array to Binary Search Tree.cpp

/*
Convert Sorted Array to Binary Search Tree
==========================================

Given an integer array nums where the elements are sorted in ascending order, convert it to a height-balanced binary search tree.

A height-balanced binary tree is a binary tree in which the depth of the two subtrees of every node never differs by more than one.

Example 1:
Input: nums = [-10,-3,0,5,9]
Output: [0,-3,9,-10,null,5]
Explanation: [0,-10,5,null,-3,null,9] is also accepted:

Example 2:
Input: nums = [1,3]
Output: [3,1]
Explanation: [1,3] and [3,1] are both a height-balanced BSTs.
 
Constraints:
1 <= nums.length <= 104
-104 <= nums[i] <= 104
nums is sorted in a strictly increasing order.
*/

/**
 * 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* buildTree(vector<int>& A, int st, int en) {
      if(st > en) return NULL;
      int mid = (st + en) / 2;
      TreeNode* root = new TreeNode(A[mid]);
      root->left = buildTree(A, st, mid-1);
      root->right = buildTree(A, mid+1, en);
      return root;
    }
  
    TreeNode* sortedArrayToBST(vector<int>& A) {
      return buildTree(A, 0, A.size()-1);
    }
};