Create 3Sum.java

Author: 17tanya

TwoPointer/3Sum.java

@@ -0,0 +1,54 @@
+//LeetCode 15. 3Sum
+//Question - https://leetcode.com/problems/3sum/
+
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        int n = nums.length;
+        List<List<Integer>> res = new ArrayList<>();
+        
+        //to help facilitate finding the triplets
+        Arrays.sort(nums);
+        
+        //consider nums[i] as the first element of the triplet
+        for(int i = 0 ; i < n - 2 ; i++){
+            //the array is sorted, if the first element is greater than 0 then all 
+            //other elements after the current element would be positive as well.
+            //The sum can never be 0.
+            if(nums[i] > 0) break;
+            
+            //skip the current number if it has already been used as the first element
+            if(i > 0 && nums[i] == nums[i-1]) continue;
+            
+            
+            //logic to find the other two elements of the triplets
+            int low = i+1;
+            int high = n - 1;
+            while(low < high){
+                int sum = nums[i] + nums[low] + nums[high];
+                
+                //if sum of the chosen elements is < 0
+                if(sum < 0) low++;
+                //if sum of the chosen elements is > 0
+                else if(sum > 0) high--;
+                //the sum is 0
+                else{
+                    //append the current triplet
+                    res.add(Arrays.asList(nums[i], nums[low], nums[high]));
+                    
+                    //keeping the first element fixed(i) and exploring other numbers
+                    //for the other two positions in the triplet
+                    //loops to avoid duplicates
+                    while(low < high && nums[low] == nums[low + 1]) low++;
+                    while(low < high && nums[high] == nums[high - 1]) high--;
+                    
+                    //need to update because after the loop the pointers point to last 
+                    //location of the repeated number
+                    low++;
+                    high--;
+                }
+            }
+            
+        }
+        return res;
+    }
+}