Need something to compare with like for finding largest sum of sub array problem, first consider the sum of k elements to start comparing with.
The second pointer in this problem is starting from k in the first loop itself. In case there are constraints like the matching sub string needs to be at-least 3 char long that can be used as initial value of k
// Javascript code for // O(n) solution for finding // maximum sum of a subarray // of size k function maxSum(arr, n, k) { let max = 0; let sum = 0; // find initial sum of first k elements for (let i = 0; i < k; i++) { sum += arr[i]; max = sum; } // iterate the array once // and increment the right edge for (let i = k; i < n; i++) { sum += arr[i] - arr[i - k]; // compare if sum is more than max, // if yes then replace // max with new sum value if (sum > max) { max = sum; } } return max; }// Driver codelet arr = [1, 4, 2, 10, 2, 3, 1, 0, 20];let k = 4;let n = arr.length;console.log(maxSum(arr, n, k));