public List<List<Integer>> threeSum(int[] num) {
    Arrays.sort(num);
    List<List<Integer>> res = new LinkedList<>();
    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) {
                    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;
}

var mysort = function(a, b) {
    return a-b;
}

var threeSum = function(ns) {
    // everything is sorted
    ns.sort(mysort);

    // acc
    let res = [];

    // loop all #, but we keep last 2 elements
    for (let i = 0; i < ns.length-2; i++) {

        // 1. i === 0, rm 1st element
        // 2. same same skip
        if (i === 0 || (i > 0 && ns[i] !== ns[i-1])) {
        //if (true) {
            // the 2nd element
            let lo = i+1;
            // the end element
            let hi = ns.length-1;
            // remove the 1st element
            let sum = 0 - ns[i];

            // bi search
            while (lo < hi) {

                console.log(lo, hi, ns[lo], ns[hi], sum)

                // bi search: 2nd element + end element === sum
                if ((ns[lo] + ns[hi]) === sum) {
                    console.log('push');
                    res.push([ns[i], ns[lo], ns[hi]]);

                    // skip: lo < hi, lo skip equal
                    while (lo < hi && ns[lo] === ns[lo+1]) lo++;

                    // skip: lo < hi, hi skip equal
                    while (lo < hi && ns[hi] === ns[hi-1]) hi--;

                    // closer
                    lo++;

                    // closer
                    hi--;
                }
                else if (ns[lo] + ns[hi] < sum)
                    lo++; // lo + hi < sum, lo++
                else
                    hi--; // lo + hi > sum, hi--
            }
        }

        return res;
    }
}

[-1,0,1,2,-1,-4]