【LeetCode】3Sum

Given an array S of n integers, are there elements a, b, c 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) {
        ArrayList<ArrayList<Integer>> re = new ArrayList<ArrayList<Integer>>(); 
        Map<ArrayList<Integer>,Integer> map = new HashMap<ArrayList<Integer>,Integer>();
        ArrayList<Integer> ai = new ArrayList<Integer>();
        if(num.length<=2)
            return re;
        Arrays.sort(num);
        for(int i=0;i<num.length-2;i++){
            int temp = num[i];
            ai = new ArrayList<Integer>();
            ai.add(temp);
            int start = i+1;
            int end=num.length-1;
            int target = -temp;
            while(end>start){
                if(num[start]+num[end]==target){
                    ai.add(num[start]);
                    ai.add(num[end]);
                    map.put(ai, 1);
                    ai=new ArrayList<Integer>();
                    ai.add(temp);
                    start++;
                    end--;
                    continue;
                }
                if(num[start]+num[end]>target){
                    end--;
                    continue;
                }
                if(num[start]+num[end]<target){
                    start++;
                    continue;
                }
            }
        }
        re.addAll(map.keySet());
        return re;
        
    }
}