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));
 	}
 }