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[leetcode] 4Sum
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)思路：
与3Sum 一样，只是多了一个数字，所以多了一层循环。
题解：
class Solution {public: vector<vector<int> > fourSum(vector<int> &num, int target) { vector<vector<int> > res; vector<int> tmp; sort(num.begin(), num.end()); for(int i=0;i<num.size();i++) { while(i>0 && num[i]==num[i-1]) i++; for(int j=i+1;j<num.size();j++) { while(j>i+1 && num[j]==num[j-1]) j++; int l = j+1; int r = num.size()-1; while(l<r) { while(l>j+1 && num[l]==num[l-1]) l++; while(r<num.size()-1 && num[r]==num[r+1]) r--; if(l<r) { int sum = num[i]+num[j]+num[l]+num[r]; if(sum>target) r--; else if(sum<target) l++; else { tmp.clear(); tmp.push_back(num[i]); tmp.push_back(num[j]); tmp.push_back(num[l]); tmp.push_back(num[r]); res.push_back(tmp); l++; } } } } } return res; }};
[leetcode] 4Sum
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