add solution for 3sum

Author: Ujoonnee

3sum/Ujoonnee.java

@@ -0,0 +1,54 @@
+import java.util.*;
+import java.util.stream.Collectors;
+
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        Map<Integer, Integer> map = new HashMap<>();
+        for(int i = 0; i < nums.length; i++) {
+            map.put(nums[i], i);
+        }
+
+        Set<Triplet> set = new HashSet<>();
+        for(int i = 0; i < nums.length; i++) {
+            int target = -nums[i];
+            for(int j = i+1; j < nums.length; j++) {
+                int operand = target - nums[j];
+                if (map.containsKey(operand) && map.get(operand) > j) {
+                    set.add(new Triplet(nums[i], nums[j], operand));
+                }
+            }
+        }
+
+        return set.stream().map(t -> List.of(t.triplet[0], t.triplet[1], t.triplet[2])).collect(Collectors.toList());
+    }
+}
+
+class Triplet {
+    int[] triplet;
+
+    Triplet(int first, int second, int third) {
+        this.triplet = new int[] { first, second, third };
+        Arrays.sort(triplet);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (!(o instanceof Triplet that)) {
+            return false;
+        }
+
+        for(int i = 0; i<3; i++) {
+            if (this.triplet[i] != that.triplet[i]) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(triplet[0], triplet[1], triplet[2]);
+    }
+}