feat: 3sum

Author: minji-go

3sum/minji-go.java

@@ -1,41 +1,30 @@
-/*
-    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 70ms
-    Space Complexity: O(N), Memory 51.63MB
-*/
+/**
+ * <a href="https://leetcode.com/problems/3sum/">week02-4.3sum</a>
+ * 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 2021ms
+ * Space Complexity: O(N), Memory 53.9MB
+ */
+
 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);
+        Map<Integer, Integer> map = new HashMap<>();
+        for(int i=0; i<nums.length; i++){
+            map.put(nums[i], i);
         }
 
-        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++) {
-            if(i>0 && nums[i]==nums[i-1]) continue;
-
-            for(int j=i+1; j<nums.length; j++){
-                if(j>i+1 && nums[j]==nums[j-1]) continue;
-
-                int target = -(nums[i]+nums[j]);
-                if(nums[j]>target) continue;
-
-                int count = number.getOrDefault(target,0);
-                if(nums[i]==target) count--;
-                if(nums[j]==target) count--;
-                if(count<=0) continue;
-
-                List<Integer> triplet = List.of(nums[i], nums[j], target);
-                if(triplet.equals(lastTriplet)) continue;
-                lastTriplet = triplet;
-                triplets.add(triplet);
+        Set<List<Integer>> triplets = new HashSet<>();
+        for(int i=0; i<nums.length-2; i++){
+            for(int j=i+1; j<nums.length-1; j++){
+                int sum = nums[i]+nums[j];
+                if(map.containsKey(-sum) && map.get(-sum) > j){
+                    List<Integer> list = Arrays.asList(nums[i],nums[j],-sum);
+                    Collections.sort(list);
+                    triplets.add(list);
+                }
             }
         }
-        return triplets;
+
+        return new ArrayList<>(triplets);
     }
 }