4Sum

struct TwoSum{
public:
    TwoSum(int idx1, int idx2, int v) :
        index1(idx1), index2(idx2), value(v) {}
public:
    int index1;
    int index2;
    int value;
};

struct increaing {
    bool operator() (TwoSum ts1, TwoSum ts2) {
        if (ts1.value == ts2.value){
            if (ts1.index1 == ts1.index1) return (ts1.index2 < ts2.index2);
            else return (ts1.index1 < ts2.index1);
        }
        else
            return (ts1.value < ts2.value);
    }
};

class Solution {
public:
    int find_start_index(vector<TwoSum>& twoSums, int begin, int end, int val){
        while (begin <= end) {
            int mid = begin + (end - begin) / 2;
            if (twoSums[mid].value >= val) end = mid - 1;
            else begin = mid + 1;
        }
        return end + 1;
    };

    int find_end_index(vector<TwoSum>& twoSums, int begin, int end, int val){
        while (begin <= end) {
            int mid = begin + (end - begin) / 2;
            if (twoSums[mid].value <= val) begin = mid + 1;
            else end = mid - 1;
        }
        return begin - 1;
    }

    vector<vector<int> > fourSum(vector<int> &num, int target) {
        // calculate sum for every two elements and sort
        vector<TwoSum> twoSums;
        for (size_t i = 0; i < num.size(); i++){
            for (size_t j = i + 1; j < num.size(); j++) {
                twoSums.push_back(TwoSum(i, j, num[i] + num[j]));
            }
        }
        sort(twoSums.begin(), twoSums.end(), increaing());

        vector<vector<int> > result;
        for (size_t i = 0; i < twoSums.size(); i++){
            size_t begin, end;
            int val = target - twoSums[i].value;

            begin = find_start_index(twoSums, i + 1, twoSums.size() - 1, val);
            end = find_end_index(twoSums, i + 1, twoSums.size() - 1, val);

            for (size_t j = begin; j <= end; j++) {
                if (twoSums[j].index1 == twoSums[i].index1) continue;
                if (twoSums[j].index2 == twoSums[i].index1) continue;
                if (twoSums[j].index1 == twoSums[i].index2) continue;
                if (twoSums[j].index2 == twoSums[i].index2) continue;

                vector<int> tmp;
                tmp.push_back(num[ twoSums[i].index1]);
                tmp.push_back(num[ twoSums[i].index2]);
                tmp.push_back(num[ twoSums[j].index1]);
                tmp.push_back(num[ twoSums[j].index2]);
                sort(tmp.begin(), tmp.end());

                if (find(result.begin(), result.end(), tmp) == result.end())
                    result.push_back(tmp);
            }
        }
        return result;
    };
};