[LeetCode] #18 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)

 本文如果先确定一个数，可以将问题转化为3Sum问题，3Sum问题解法如：http://www.cnblogs.com/Scorpio989/p/4440236.html

时间：180ms。代码如下：

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