Leetcode problem 39 - Left and Right Sum Differences

Given a 0-indexed integer array nums, find a 0-indexed integer array answer where:

answer.length == nums.length. answer[i] = |leftSum[i] - rightSum[i]|. Where:

leftSum[i] is the sum of elements to the left of the index i in the array nums. If there is no such element, leftSum[i] = 0. rightSum[i] is the sum of elements to the right of the index i in the array nums. If there is no such element, rightSum[i] = 0. Return the array answer.

Example 1:

Input: nums = [10,4,8,3] Output: [15,1,11,22] Explanation: The array leftSum is [0,10,14,22] and the array rightSum is [15,11,3,0]. The array answer is [|0 - 15|,|10 - 11|,|14 - 3|,|22 - 0|] = [15,1,11,22]. Example 2:

Input: nums = [1] Output: [0] Explanation: The array leftSum is [0] and the array rightSum is [0]. The array answer is [|0 - 0|] = [0].

/**
 * @param {number[]} nums
 * @return {number[]}
 */
var leftRightDifference = function (nums) {
  let leftSum = [];
  let rightSum = [];
  for (let i = 0; i < nums.length; i++) {
    if (i === 0) {
      leftSum[i] = 0;
    } else {
      leftSum[i] = leftSum[i - 1] + nums[i - 1];
    }
  }
  for (let i = nums.length - 1; i >= 0; i--) {
    if (i === nums.length - 1) {
      rightSum[i] = 0;
    } else {
      rightSum[i] = rightSum[i + 1] + nums[i + 1];
    }
  }
  for (let i = 0; i < nums.length; i++) {
    nums[i] = Math.abs(leftSum[i] - rightSum[i]);
  }
  return nums;
};

let leftSum = []; and let rightSum = [];: Initialize two empty arrays, leftSum and rightSum, which will be used to store the cumulative sums of elements to the left and right of each element in the nums array, respectively.

First Loop (Calculating leftSum):

for (let i = 0; i < nums.length; i++) {: Start a loop that iterates over each element in the nums array. if (i === 0) {: Check if the current index is 0 (first element). leftSum[i] = 0: For the first element, set its leftSum to 0. leftSum[i] = leftSum[i - 1] + nums[i - 1]: For other elements, calculate the cumulative sum by adding the previous element’s leftSum value and the corresponding element from the nums array.

Second Loop (Calculating rightSum):

for (let i = nums.length - 1; i >= 0; i--) {: Start a loop that iterates over each element in the nums array in reverse order. if (i === nums.length - 1) {: Check if the current index is the last index (last element). rightSum[i] = 0: For the last element, set its rightSum to 0. rightSum[i] = rightSum[i + 1] + nums[i + 1]: For other elements, calculate the cumulative sum by adding the next element’s rightSum value and the corresponding element from the nums array.

Third Loop (Calculating absolute differences and modifying nums):

for (let i = 0; i < nums.length; i++) {: Start a loop that iterates over each element in the nums array. nums[i] = Math.abs(leftSum[i] - rightSum[i]): Calculate the absolute difference between leftSum[i] and rightSum[i] and store the result back in the nums array. return nums;: Return the modified nums array containing the absolute differences.