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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]]
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class Solution{public: vector<vector<int>> fourSum(vector<int> & nums,int target){ vector<vector<int>> res; if(nums.size()<4) return res; sort(nums.begin(),nums.end()); unordered_map<int,vector<pair<int,int>>> cache; for(size_t i =0 ;i< nums.size();i++){ for (size_t j = i+ 1;j<nums.size();j++){ cache[nums[i]+nums[j]].push_back(pair<int,int>{i,j}); } } for(size_t i=0;i<nums.size();i++){ for (size_t j = i+1;j<nums.size();j++){ int key = target - nums[i] - nums[j]; if(cache.find(key) == cache.end()){ continue; } vector<pair<int,int>> vec = cache[key]; for(size_t k = 0;k < vec.size();k++){ if(vec[k].first <= j) continue; res.push_back({nums[vec[k].first],nums[vec[k].second],nums[i],nums[j]}); } } } sort(res.begin(),res.end()); res.erase(unique(res.begin(),res.end()),res.end()); return res; }};
4Sum
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