Pivot Index

Calculate the pivot index of this array. The pivot index is the index where the sum of all the numbers strictly to the left of the index is equal to the sum of all the numbers strictly to the index's right. Return the leftmost pivot index. If no such index exists, return -1. Source

package questions

import _utils.UseCommentAsDocumentation
import utils.shouldBe

/**
 * Calculate the pivot index of this array.
 * The pivot index is the index where the sum of all the numbers strictly to the left of the index
 * is equal to the sum of all the numbers strictly to the index's right.
 * Return the leftmost pivot index. If no such index exists, return -1.
 * [Source](https://leetcode.com/problems/pivot-index/)
 */
@UseCommentAsDocumentation
private fun pivotIndex(nums: IntArray): Int {
    val cumulativeSumLeft = IntArray(nums.size) { 0 }
    val cumulativeSumRight = IntArray(nums.size) { 0 }
    for (i in 0..nums.lastIndex) {
        cumulativeSumLeft[i] = (cumulativeSumLeft.getOrNull(i - 1) ?: 0) + nums[i]
        cumulativeSumRight[nums.lastIndex - i] =
            (cumulativeSumRight.getOrNull(nums.lastIndex - i + 1) ?: 0) + nums[nums.lastIndex - i]
    }
    for (i in 0..nums.lastIndex) {
        if (cumulativeSumRight[i] == cumulativeSumLeft[i]) {
            return i
        }
    }
    return -1
}

fun main() {
    pivotIndex(intArrayOf(1, 7, 3, 6, 5, 6)) shouldBe 3
    pivotIndex(intArrayOf(1, 2, 3)) shouldBe -1
    pivotIndex(intArrayOf(2, 1, -1)) shouldBe 0
}

---

View on GitHub

Updated on 2021-11-16