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class Solution {
    public List<List<Integer>> threeSum(int[] num) {
        Arrays.sort(num);
        List<List<Integer>> res = new ArrayList<>(); 
        for (int i = 0; i < num.length-2; i++) {
            if (i == 0 || (i > 0 && num[i] != num[i-1])) {
                int lo = i+1, hi = num.length-1, sum = 0 - num[i];
                while (lo < hi) {
                    if (num[lo] + num[hi] == sum) {
                        // List<Integer> temp = new ArrayList<>();
                        // temp.add(num[lo]);
                        // temp.add(num[i]);
                        // temp.add( num[hi]);
                        // res.add(temp);
                     // one line alternative
                        res.add(Arrays.asList(num[i], num[lo], num[hi]));
                        while (lo < hi && num[lo] == num[lo+1]) lo++;
                        while (lo < hi && num[hi] == num[hi-1]) hi--;
                        lo++; hi--;
                    } else if (num[lo] + num[hi] < sum) lo++;
                    else hi--;
               }
            }
        }
        return res;
    }
}