[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: 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]
]

解题思路

本题是3sum的升级版，基本思路完全一样，只是在外围多加了一层循环，需要注意的是c++中，vector的size()函数返回的值是unsigned类型，因此不可以直接-3，需要做类型的转换，否则会报错：Runtime Error Message: reference binding to null pointer of type 'struct value_type'

具体代码如下：

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        sort(nums.begin(),nums.end());
        set<vector<int> > res;
        
        int number = nums.size();
        for(int i = 0; i < number - 3; ++i)
        {
            for(int j = i + 1; j < number - 2; ++j)
            {
                int l = j + 1, r = nums.size() - 1;
                int tar = target - nums[i] - nums[j];
                while(l < r)
                {
                    if(nums[l] + nums[r] < tar)
                        l++;
                    else if(nums[l] + nums[r] > tar)
                        r--;
                    else
                    {
                        res.insert({nums[i],nums[j],nums[l], nums[r]});
                        l++; r--;
                     }
                }
            }
        }
        return vector<vector<int> >(res.begin(),res.end());
    }
};