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[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)
分析：在3Sum基础上稍做改动写的程序class Solution {public: vector<vector<int> > fourSum(vector<int> &num, int target) { vector<vector<int> > result; int len = num.size(); if(len<4) return result; int sum; sort(num.begin(),num.end()); for(int mid2 = 2;mid2<len-1;mid2++){ for(int mid = 1;mid<mid2;mid++){ int low = 0,up = len-1; while(low<mid && mid2<up){ if(low < mid-1 && low>0 && num[low] == num[low-1]){ low++; continue; } if(up > mid2+1 && up<len-1 && num[up] == num[up+1]){ up--; continue; } sum = num[low] + num[mid] + num[mid2] + num[up]; if(sum == target){ vector<int> temp; temp.push_back(num[low]); temp.push_back(num[mid]); temp.push_back(num[mid2]); temp.push_back(num[up]); if(find(result.begin(),result.end(),temp)== result.end()) result.push_back(temp); }else if(sum>target){ up--; continue; }else { low++; continue; } low++; up--; }//end while }//end for }//end for return result; }};
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