Leetcode: 3Sum

Given an array S of n integers, are there elements abc in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

  • Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
  • The solution set must not contain duplicate triplets.
    For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)
public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<ArrayList<Integer>> output = new ArrayList<ArrayList<Integer>>();
        if(num==null||num.length<3){
            return output;
        }
        HashSet<ArrayList<Integer>> pairsFound = new HashSet<ArrayList<Integer>>();
        Arrays.sort(num);
        for(int i=0;i<num.length;i++){
            ArrayList<ArrayList<Integer>> result = twoSum(num, 0-num[i]);
            for(ArrayList<Integer> pair:result){
                if(pair.get(0)!=i && pair.get(1)!=i){
                    int[] temp = new int[3];
                    temp[0] = i;
                    temp[1] = pair.get(0);
                    temp[2] = pair.get(1);
                    Arrays.sort(temp);
                    ArrayList<Integer> newTriple = new ArrayList<Integer>();
                    for(int j=0;j<temp.length;j++){
                        newTriple.add(num[temp[j]]);
                    }
                    if(!pairsFound.contains(newTriple)){
                        pairsFound.add(newTriple);
                        output.add(newTriple);
                    }
                }
            }
        }
        return output;
    }

    public ArrayList<ArrayList<Integer>> twoSum(int[] num, int sum){
        ArrayList<ArrayList<Integer>> output = new ArrayList<ArrayList<Integer>>();
        int i = 0;
        int j = num.length-1;
        while(i<j){
            if(num[i]+num[j]==sum){
                ArrayList<Integer> newPair = new ArrayList<Integer>();
                newPair.add(i);
                newPair.add(j);
                output.add(newPair);
                i++;
                j--;
            }else if(num[i]+num[j]>sum){
                j--;
            }else{
                i++;
            }
        }
        return output;
    }
}
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