max-subarray-sum.js

// Sliding Window - maxSubarraySum
// Given an array of integers and a number, write a function called maxSubarraySum,
// which finds the maximum sum of a subarray with the length of the number passed to the function.

// Note that a subarray must consist of consecutive elements from the original array.
// In the first example below, [100, 200, 300] is a subarray of the original array, but [100, 300] is not.
// maxSubarraySum([100,200,300,400], 2) // 700

// Constraints:
// Time Complexity - O(N)
// Space Complexity - O(1)

function maxSubarraySum(arr, num) {
  if (arr.length < num) return null;

  let tempSum = 0;

  for (let i = 0; i < num; i++) {
    tempSum += arr[i];
  }

  let maxSum = tempSum;

  for (let i = 0; i < arr.length - num; i++) {
    tempSum = tempSum - arr[i] + arr[i + num];
    maxSum = Math.max(maxSum, tempSum);
  }

  return maxSum;
}

console.log(maxSubarraySum([1, 4, 2, 10, 23, 3, 1, 0, 20], 4)); // 39
console.log(maxSubarraySum([3, -2, 7, -4, 1, -1, 4, -2, 1], 2)); // 5
console.log(maxSubarraySum([2, 3], 3)); // null