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 List<List<Integer>> threeSum(int[] num) {
        HashSet<List<Integer>> set = new HashSet<List<Integer>>();
        int len=num.length;
        Arrays.sort(num);
        for (int i = 0; i < len-2; i++) {
            if (num[i]>0)break;
            for (int j = len-1 ; j >i+1 ; j--) {
                if (num[j]<0) break;
                int ab = num[i] + num[j];
                int c = -ab;
                int k = bs(c, num, i + 1, j - 1);
                if (k>0) {
                    ArrayList<Integer> list = new ArrayList<Integer>();
                    list.add(num[i]);
                    list.add(num[k]);
                    list.add(num[j]);
                    set.add(list);
                }
            }
        }
        return new ArrayList<List<Integer>>(set);
    }
        int bs(int c, int[] num, int l, int r) {
        if(num[l] > c || num[r] < c) return -1;
        while(l <= r) {
            int m = (l+r)/2;
            if(num[m] == c) return m;
            else if(num[m] < c) l = m+1;
            else r = m-1;
        }
        return -1;
    }
}