Add 560. Subarray Sum Equals K

Author: manojbaishya

README.md

@@ -13,6 +13,34 @@ Computer Science problems in Java.
 
 1. LLD of chess
 
+### Patterns
+
+1. Two Pointers
+2. Sliding Window
+3. Prefix Sums
+4. Merge Intervals
+5. Binary Search (and Variants)
+6. Sorting-Based Patterns
+7. Fast and Slow Pointers
+8. Backtracking & Recursive Search
+9. Divide and Conquer
+10. Linked List Techniques (Dummy Node, In-place Reversal)
+11. Stacks and Queues
+12. Monotonic Stack / Queue
+13. Expression Evaluation (Two Stacks)
+14. String Manipulation & Regular Expressions
+15. Hashmaps & Frequency Counting
+16. Binary Trees & BSTs (Traversal, Construction, Properties)
+17. Path Sum & Root-to-Leaf Techniques
+18. Kth Largest/Smallest Elements (Heaps / QuickSelect)
+19. Top K Frequent Elements
+20. Merge K Sorted Lists
+21. Dynamic Programming (Including Knapsack, Range DP, etc.)
+22. Greedy & Interval Partitioning
+23. Graph Traversals (BFS, DFS)
+24. Graph Algorithms (DAGs, MSTs, Shortest Paths, etc.)
+25. Design Problems (LRU Cache, Twitter, etc.)
+
 ## Question Bank
 
 **𝗔𝗿𝗿𝗮𝘆𝘀 𝗮𝗻𝗱 𝗦𝘁𝗿𝗶𝗻𝗴𝘀:**
@@ -267,34 +295,6 @@ Computer Science problems in Java.
     - Efficient for problems with optimal substructure
     - Covers tasks like activity selection, minimum coins
 
-### More patterns
-
-1. Two Pointers
-2. Sliding Window
-3. Prefix Sums
-4. Merge Intervals
-5. Binary Search (and Variants)
-6. Sorting-Based Patterns
-7. Fast and Slow Pointers
-8. Backtracking & Recursive Search
-9. Divide and Conquer
-10. Linked List Techniques (Dummy Node, In-place Reversal)
-11. Stacks and Queues
-12. Monotonic Stack / Queue
-13. Expression Evaluation (Two Stacks)
-14. String Manipulation & Regular Expressions
-15. Hashmaps & Frequency Counting
-16. Binary Trees & BSTs (Traversal, Construction, Properties)
-17. Path Sum & Root-to-Leaf Techniques
-18. Kth Largest/Smallest Elements (Heaps / QuickSelect)
-19. Top K Frequent Elements
-20. Merge K Sorted Lists
-21. Dynamic Programming (Including Knapsack, Range DP, etc.)
-22. Greedy & Interval Partitioning
-23. Graph Traversals (BFS, DFS)
-24. Graph Algorithms (DAGs, MSTs, Shortest Paths, etc.)
-25. Design Problems (LRU Cache, Twitter, etc.)
-
 ## Roadmap:
 
 ✅Build a strong foundation (WEEKS 1-4)

src/main/java/org/dojo/leetcode/ArrayProblems.java

@@ -627,8 +627,8 @@ public List<Integer> spiralOrder(int[][] matrix) {
 
         int top = 0, bottom = m - 1, left = 0, right = n - 1;
 
-        while(list.size() != m * n) {
-            for (int c = left; c <  right; c++) list.add(matrix[top][c]);
+        while (list.size() != m * n) {
+            for (int c = left; c < right; c++) list.add(matrix[top][c]);
             for (int r = top; r <= bottom; r++) list.add(matrix[r][right]);
             if (top == bottom) break;
             for (int c = right - 1; c >= left; c--) list.add(matrix[bottom][c]);
@@ -643,4 +643,19 @@ public List<Integer> spiralOrder(int[][] matrix) {
 
         return list;
     }
+
+    public int subarraySum(int[] nums, int k) {
+        int prefixSum = 0;
+        int count = 0;
+
+        Map<Integer, Integer> sums = new HashMap<>();
+        sums.put(prefixSum, 1);
+        for (int num : nums) {
+            prefixSum += num;
+            int prevSum = prefixSum - k;
+            if (sums.containsKey(prevSum)) count += sums.get(prevSum);
+            sums.merge(prefixSum, 1, Integer::sum);
+        }
+        return count;
+    }
 }

src/test/java/org/dojo/leetcode/ArrayProblemsTests.java

@@ -418,4 +418,18 @@ void spiralOrder(int[][] matrix, List<Integer> expected) {
         System.out.printf("Expected: %s, Actual: %s%n", expected, actual);
         assertEquals(expected, actual);
     }
+
+    static Stream<Arguments> subarraySumData() {
+        return Stream.of(
+                Arguments.of(new int[] {1, 1, 1}, 2, 2),
+                Arguments.of(new int[] {1, 2, 3}, 3, 2)
+        );
+    }
+    @ParameterizedTest
+    @MethodSource("subarraySumData")
+    void subarraySum(int[] nums, int k, int expected) {
+        int actual = sut.subarraySum(nums, k);
+        System.out.printf("Expected: %d, Actual: %d%n", expected, actual);
+        assertEquals(expected, actual);
+    }
 }