3sum

3sum.java

import java.util.Arrays;
class Solution {
    

    public List<List<Integer>> threeSum(int[] nums) {
        int sum=0;
        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        
        Arrays.sort(nums);
        
        for(int i=0; i < nums.length - 1; i++){
            
            if(i>0 && nums[i] == nums[i-1]) continue;
            
            HashSet<Integer> set = new HashSet<Integer>();
            for(int j = i+1; j < nums.length; j++){
                
                int searched = sum -  (nums[i] + nums[j]);
                
                //look for the element already examined and pushed to set.(think this 'searched' integer as the third integer)
                if(set.contains(searched)){                   
                    List<Integer> list = new ArrayList<Integer>();
                    list.add(nums[i]);
                    list.add(searched);
                    list.add(nums[j]);
                    ret.add(list);
                    
                    //skip same numbers to prevent duplicate founds.
                    while (j + 1 < nums.length && nums[j+1] == nums[j]) ++j;
                }
                else{
                    //push traversed integer to set. In other words, convert array to hash one by one
                    set.add(nums[j]);
                }                                
            }            
        }
        
        return ret;
       
    }   
}