class Solution {public:    vector<vector<int> > threeSum(vector<int> &num) {        vector<vector<int> > ret;        if (num.size() < 3) return ret;        std::sort(num.begin(), num.end());        for (int i0 = 0; i0 < num.size() - 2; i0++)        {            if (i0 > 0 && num[i0] == num[i0 - 1]) continue;            int tgt = -num[i0];            int i1 = i0 + 1, i2 = num.size() - 1;            while (i1 < i2)            {                int curr = num[i1] + num[i2];                if (curr == tgt)                {                    vector<int> rt;                    rt.push_back(num[i0]);                    rt.push_back(num[i1]);                    rt.push_back(num[i2]);                    ret.push_back(rt);                    while (++i1, num[i1] == num[i1-1]);                }                else if (curr < tgt)                {                    while (++i1, num[i1] == num[i1-1]);                }                else if (curr > tgt)                {                    while (--i2, num[i2] == num[i2 + 1]);                }            }        }        return ret;    }};