【LeetCode】16. 4Sum

题目：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.

给定一个数组，找出所有4个数的组合使得它们的和为target

思路：和3Sum，Two Sum都是类似的。主要是两个点：

1. 对数组排序，然后双指针分别从头尾遍历

2. 注意去重的问题

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        int nsize = nums.size();
        sort(nums.begin(), nums.end());
        vector<vector<int>> result;
        for(int i = 0; i < nsize; i++){
            if(i != 0 && nums[i] == nums[i - 1])
                continue;
            for(int j = i + 1; j < nsize; j++){
                if(j != i + 1 && nums[j] == nums[j - 1])
                    continue;
                int p = j + 1, q = nsize - 1;
                while(p < q){
                    int sum = nums[i] + nums[j] + nums[p] + nums[q];
                    if(sum < target){
                        ++p;
                        continue;
                    }
                    if(sum > target){
                        --q;
                        continue;
                    }
                   
                        vector<int> tmp;
                        tmp.push_back(nums[i]);
                        tmp.push_back(nums[j]);
                        tmp.push_back(nums[p]);
                        tmp.push_back(nums[q]);
                        result.push_back(tmp);
                        
                        while(++p < q && nums[p] == nums[p - 1]);
                        while(p < --q && nums[q] == nums[q + 1]);
                }
            }
        }
        return result;
    }
};