4Sum

跟求3sum差不多

class Solution {
public:
     vector > fourSum(vector &num, int target) {
        vector > result;
        sort(num.begin(), num.end());
        for(int i = num.size() - 1; i > 2; i--) {
            if(i == num.size() - 1 || num[i] != num[i + 1]) {
                vector > tmp = threeSum(num, i - 1, target - num[i]);
                for(int k = 0; k < tmp.size(); k++) {
                    tmp[k].push_back(num[i]);
                    result.push_back(tmp[k]);
                }
            }
        }
        return result;
    }
private:
       vector > threeSum(vector &num, int end, int target) {
        int pre = -1;
        vector > result;
        for(int i = 0; i <= end; i++) {
            int temp = num[i];
            if(pre != -1 && num[pre] == num[i]) {
                continue;
            }
            pre = i;
            int low = i + 1, high = end;
            int val = target - num[i];
            while(low < high) {
                int t = num[low] + num[high];
                if(t == val) {
                    vector tmp;
                    tmp.push_back(num[i]);
                    tmp.push_back(num[low]);
                    tmp.push_back(num[high]);
                    result.push_back(tmp);
                    low++;
                    high--;
                    while(low < high && num[low] == num[low - 1]) {
                        low++;
                    }
                    while(low < high && num[high] == num[high + 1]) {
                        high--;
                    }
                } else if(t < val) {
                    low++;
                } else {
                    high--;
                }
            }
        }
        return result;
    }
};