4Sum

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
• The solution set must not contain duplicate quadruplets.

    For example, given array S = {1 0 -1 0 -2 2}, and target = 0.

    A solution set is:
    (-1,  0, 0, 1)
    (-2, -1, 1, 2)
    (-2,  0, 0, 2)

思路：
类似前3题，不过在外层是两层循环。要注意判断重复的情况。
代码：

    vector<vector<int> > fourSum(vector<int> &num, int target) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        vector<vector<int> > result;
        vector<int> tmp;
        result.clear();
        tmp.clear();
        int i,j,k,t;
        int len = num.size();
        if(len < 4)
            return result;
        sort(num.begin(), num.end());
        for(i = 0; i < len-3; i++){
            while(i > 0 && num[i] == num[i-1])
                i++;
            for(j = i+1; j < len-2; j++){
                while(j > i+1 && num[j] == num[j-1])
                    j++;                    
                k = j+1;
                t = len-1;
                while(k<t){
                    while(k > j+1 && num[k] == num[k-1])
                        k++;
                    while(t < len-1 && num[t] == num[t+1])
                        t--;
                    if(k>=t)
                        break;
                    int total = num[i] + num[j] + num[k] + num[t];
                    if(total < target)
                        k++;
                    else if(total > target)
                        t--;
                    else{
                        tmp.push_back(num[i]);
                        tmp.push_back(num[j]);
                        tmp.push_back(num[k]);
                        tmp.push_back(num[t]);
                        result.push_back(tmp);
                        tmp.clear();
                        k++;
                        t--;
                    }
                }
            }
        }
        return result;
    }