optimal- finds max sum of subarray of window size K, O(N) Time, O(1) space

Author: NirmalSilwal

GeeksForGeeks/SlidingWindow/MaximumSubarraySum.java

@@ -2,26 +2,55 @@
 
 public class MaximumSubarraySum {
 
-// brute force approch - O(N^2) Time | O(1) Space
+// brute force approch - O(N*k) Time | O(1) Space
 public static int maxSubarraySum(int[] arr, int k) {
 
 int maxsum = Integer.MIN_VALUE;
 
 for (int i = 0; i <= arr.length - k; i++) {
-int currsum = 0;
+int windowsum = 0;
 
 for (int window = 0; window < k; window++) {
-currsum += arr[i + window];
+windowsum += arr[i + window];
 }
-maxsum = Math.max(maxsum, currsum);
+maxsum = Math.max(maxsum, windowsum);
+}
+
+return maxsum;
+}
+
+// O(N) Time | O(1) Space
+public static int maxSubarraySum2(int[] arr, int k) {
+
+int maxsum = Integer.MIN_VALUE;
+int head = 0, tail = k, n = arr.length;
+
+int windowSum = 0;
+for (int i = 0; i < k; i++) {
+windowSum += arr[i];
+}
+maxsum = Math.max(maxsum, windowSum);
+
+for (int slide = 1; slide <= n - k; slide++) {
+
+if (head < n && tail < n) {
+windowSum -= arr[head];
+windowSum += arr[tail];
+head++;
+tail++;
+}
+maxsum = Math.max(maxsum, windowSum);
 }
 
 return maxsum;
 }
 
 public static void main(String[] args) {
 int[] arr = { 1, 4, 2, 10, 23, 3, 1, 0, 20 };
+// int[] arr = { 100, 200, 300, 400 };
 int windowSize = 4;
+
 System.out.println(maxSubarraySum(arr, windowSize));
+System.out.println(maxSubarraySum2(arr, windowSize));
 }
 }