Merge pull request matthewsamuel95#260 from arpitpandey0209/master

Added Java solution for 4-Sum problem in O(n^2Logn). Issue matthewsamuel95#256

Author: matthewsamuel95

Hashing/4_Sum/FourSum.java

@@ -0,0 +1,59 @@
+import java.util.*;
+
+public class FourSum {
+
+    static class Pair {
+        int first, second;
+        Pair(int first, int second) {
+            this.first = first;
+            this.second = second;
+        }
+    }
+
+    /*
+     *  Returns a list containing 4 numbers present at different indexes which sum to target. If there are multiple solutions, 
+     *  return any solution.
+     *  If there is no solution, return an empty list.
+     */
+    static List<Integer> findSubset(int[] nums, int target) {
+        List<Integer> result = new ArrayList<>();
+        
+        // For every pair of elements, store their sum
+        Map<Integer, Pair> map = new HashMap<>();
+        
+        int n = nums.length;
+        for (int i=0; i<n; i++) {
+            for (int j=i+1; j<n; j++) {
+                map.put(nums[i]+nums[j], new Pair(i, j));
+            }
+        }
+        
+        for (int i=0; i<n; i++) {
+            for (int j=i+1; j<n; j++) {
+                int remainingSum = target - (nums[i] + nums[j]);
+                if (map.containsKey(remainingSum)) {
+                    Pair  pair = map.get(remainingSum);
+                    if (pair.first != i && pair.first != j && pair.second != i && pair.second != j) {
+                        result.addAll(Arrays.asList(nums[i], nums[j], nums[pair.first], nums[pair.second]));
+                        return result;
+                    }
+                }
+            }
+        }
+        return new ArrayList<Integer>();
+    }
+
+    public static void main(String[] args) {
+        int target = 23;
+        int[] nums = {10, 2, 3, 4, 5, 9, 7, 8};
+        List<Integer> result = findSubset(nums, target);
+        if (result.size() != 4) {
+            System.out.println("Four elements with the given sum not found in the array!");
+        } else {
+            for (int num : result) {
+                System.out.print(num + " ");
+            }
+            System.out.println();
+        }
+    }
+}