feat: 3sum

Author: minji-go

3sum/minji-go.java

@@ -0,0 +1,37 @@
+/*
+    Problem: https://leetcode.com/problems/3sum/
+    Description: return all the triplets (i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0)
+    Concept: Array, Two Pointers, Sorting
+    Time Complexity: O(N²), Runtime 1107ms
+    Space Complexity: O(N), Memory 54.20MB
+*/
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        Map<Integer, Integer> number = new HashMap<>();
+        for(int i=0; i<nums.length; i++) {
+            number.put(nums[i], number.getOrDefault(nums[i], 0)+1);
+        }
+
+        Arrays.sort(nums);
+        Set<List<Integer>> set = new HashSet<>();
+        List<List<Integer>> triplets = new ArrayList<>();
+        List<Integer> lastTriplet = null;
+        for(int i=0; i<nums.length-1; i++) {
+            for(int j=i+1; j<nums.length; j++){
+                int twoSum = nums[i]+nums[j];
+                if(nums[j]>-twoSum) continue;
+
+                int count = number.getOrDefault(-twoSum,0);
+                if(nums[i]==-twoSum) count--;
+                if(nums[j]==-twoSum) count--;
+                if(count<=0) continue;
+
+                List<Integer> triplet = List.of(nums[i], nums[j], -twoSum);
+                int setSize = set.size();
+                set.add(triplet);
+                if(setSize != set.size()) triplets.add(triplet);
+            }
+        }
+        return triplets;
+    }
+}