Array to BST

JavaView on GFG

Intuition

Convert sorted array to height-balanced BST. Recursively use middle element as root.

Algorithm

1mid = (lo + hi) / 2. root = arr[mid]. root.left = build(lo, mid-1). root.right = build(mid+1, hi).

Common Pitfalls

•Same as LC 108. Middle element as root ensures balance.

Array to BST.java

Java

// Approach: Sort array, then recursively build BST by picking the middle element as root.
// This guarantees a height-balanced BST.
// Time: O(n log n) Space: O(n)
class Node {
    int data;
    Node left, right;

    Node(int item) {
        data = item;
        left = right = null;
    }
}

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

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

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

        int mid = start + (end - start) / 2;
        Node node = new Node(nums[mid]);
        node.left = sortedArrayToBST(nums, start, mid - 1);
        node.right = sortedArrayToBST(nums, mid + 1, end);

        return node;
    }
}

Was this solution helpful?