Convert Sorted Array to Binary Search Tree

This is NO.108 problem in leetcode.

The description of this problem is:

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.

This problem is not hard, I had a simple thought quickly. But I always made memory limit error.

For sum up, I think I need to remember some points to avoid making them again.

  1. I should make sure that I connect each node correcly, in this problem, I need to create root in my helper function as well.
  2. I need to move idx after each manipulating.
  3. I need to set the correct falg to terminal program, like min > max in this problem.

Code is below:

class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        if(nums.length == 0){
            return null;

        return createBST(nums,0, nums.length-1);

    TreeNode createBST(int[] nums, int start, int end){
            return null;
        int mid = (start+end)/2;
        TreeNode node = new TreeNode(nums[mid]);
        node.left = createBST(nums, start,mid-1);
        node.right = createBST(nums, mid+1,end);

        return node;

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