LeetCode Top Interview 150

108. Convert Sorted Array to Binary Search Tree

Easy

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.

Solution

import { TreeNode } from '../../com_github_leetcode/treenode'

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */
function sortedArrayToBST(nums: number[]): TreeNode | null {
    return makeTree(nums, 0, nums.length - 1)
}

function makeTree(nums: number[], left: number, right: number): TreeNode | null {
    if (left > right) {
        return null
    }
    const mid = Math.floor((left + right) / 2)
    const midNode = new TreeNode(nums[mid])
    midNode.left = makeTree(nums, left, mid - 1)
    midNode.right = makeTree(nums, mid + 1, right)
    return midNode
}

export { sortedArrayToBST }

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