[Leetcode] 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)

跟3Sum一样，先用两层循环，注意去重。

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
        vector<vector<int> > res;
        vector<int> v(4);
        int sum;
        if (num.size() < 4) return res;
        sort(num.begin(), num.end());
        for (int i = 0; i < num.size() - 3; ++i) {
            if (i > 0 && num[i] == num[i-1]) 
                continue;
            for (int j = i + 1; j < num.size() - 2; ++j) {
                if (j > i + 1 && num[j] == num[j-1])
                    continue;
                int low = j + 1, high = num.size() - 1;
                while (low < high) {
                    sum = num[i] + num[j] + num[low] + num[high];
                    if (sum > target) {
                        --high;
                    } else if (sum < target) {
                        ++low;
                    } else {
                        v[0] = num[i];
                        v[1] = num[j];
                        v[2] = num[low];
                        v[3] = num[high];
                        res.push_back(v);
                        while (low < num.size() && num[low+1] == num[low]) ++low;
                        while (high > 0 && num[high-1] == num[high]) --high;
                        ++low; --high;
                    }
                }
            }
        }
        return res;
    }
};