[LeetCode] Problem 108 - Convert Sorted Array to Binary Search Tree

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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:

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5
     0
    / \
  -3   9
  /   /
-10  5

Code

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public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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// O(n) runtime, O(log n) stack space
public TreeNode sortedArrayToBST(int[] nums) {
    if (nums == null || nums.length < 1)
        return null;

    return sortedArrayToBSTHelper(nums, 0, nums.length - 1);
}

private TreeNode sortedArrayToBSTHelper(int[] nums, int start, int end) {
    if (start > end)
        return null;

    if (start == end)
        return new TreeNode(nums[start]);

    int mid = (start + end) >> 1;

    TreeNode root = new TreeNode(nums[mid]);

    root.left = sortedArrayToBSTHelper(nums, start, mid - 1);
    root.right = sortedArrayToBSTHelper(nums, mid + 1, end);

    return root;
}