Added Longest Subarray Having Sum K in Java (#3241)

* Added Longest Subarray Having Sum K in Java

* Updated Java README.md

* Updated LongestSubarraySum.java to add output message

Co-authored-by: Harsh Mishra <[email protected]>

Author: neha-b2001

Java/README.md

@@ -77,6 +77,7 @@ _add list here_
 - [Implement Stack using Queue](cp/Stack_using_Queue.java)
 - [Longest Pallindrome Substring](cp/Longest_Pallindromicsubstring)
 - [N-Queens_Problem_Efficient](cp/N_Queens_Problem_Efficient.java)
+- [Longest Subarray Having Sum K](cp/LongestSubarraySum.java)
 - [Maximum of All Subarrays of Size K](cp/MaximumEachSubarray.java)
 - [Paint Fence Algorithm](cp/Paint_Fence.java)
 - [Minimum Window Substring](cp/MinimumWindowSubstring.java)

Java/cp/LongestSubarraySum.java

@@ -0,0 +1,87 @@
+/**
+ * Longest Subarray Having Sum K
+ * Given an array of integers, find the length of the longest sub-array 
+ * having sum equal to the given value K.
+ */
+
+import java.io.*;
+import java.util.*; 
+public class LongestSubarraySum {
+
+    public static int findLongestSubarray(int[] numArray, int K) {
+
+        //HashMap containing (sum, index) key:value pairs
+        HashMap<Long, Integer> hMap = new HashMap<>(); 
+
+        long sum = 0l; //Stores cumulative sum as we traverse the integer array
+        int length = 0; //Stores length of longest valid subarray
+
+        for(int i=0; i<numArray.length; i++) {
+            sum += numArray[i]; 
+
+            if(sum == K)
+                length = Math.max(length, i+1); 
+
+            //Only the first index of occurrence of a particular sum is stored,
+            //as we need to find length of longest valid subarray
+            if(!hMap.containsKey(sum)) 
+                hMap.put(sum, i); 
+
+            //Checking if HashMap contains (sum-K) as we know that:
+            // (sum - (sum - K)) = K, which is the required sum
+            if(hMap.containsKey(sum - K))
+                length = Math.max(length, i - hMap.get(sum - K)); 
+        }
+
+        return length; 
+    }
+    
+    public static void main(String[] args) throws IOException{
+        InputStreamReader read = new InputStreamReader(System.in);
+	    BufferedReader buf = new BufferedReader(read); 
+
+        //Taking input from user
+        System.out.println("Enter length of array of integers: ");
+	    int len = Integer.parseInt(buf.readLine());
+        System.out.println("Enter array of integers: ");
+	    StringTokenizer st = new StringTokenizer(buf.readLine());
+	    int numArray[] = new int[len];
+	    for(int i=0; i<len; i++) {
+	        numArray[i] = Integer.parseInt(st.nextToken()); 
+	    } 
+        System.out.println("Enter sum: ");
+	    int K = Integer.parseInt(buf.readLine()); 
+	    
+        System.out.print("Length of longest subarray having sum " + K + ": "); 
+        System.out.println(findLongestSubarray(numArray, K)); 
+    }
+
+}
+
+/*
+    Time complexity: O(N)
+    Space complexity: O(N) (for the HashMap)
+
+    TEST CASES
+
+    INPUT
+    Enter length of array of integers: 
+    8
+    Enter array of integers: 
+    2 1 3 2 1 1 1 3
+    Enter sum: 
+    4
+    OUTPUT
+    Length of longest subarray having sum 4: 3
+
+    INPUT
+    Enter length of array of integers:
+    6
+    Enter array of integers:
+    -5 8 -14 2 4 12
+    Enter sum:
+    -5
+    OUTPUT
+    Length of longest subarray having sum -5: 5
+
+ */