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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:
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)
先确定前两个数num[i],num[j]，
然后设置双指针k,l分别指向两端，往中间扫。
(1)(sum = num[i]+num[j]+num[k]+num[l]) == taget，则找到其中一个解（注意用map去重）。k++,l--.
(2)sum > target, l--
(3)sum < target, k++
class Solution { public: vector<vector<int> > fourSum(vector<int> &num, int target) { vector<vector<int> > result; if(num.empty() || num.size() < 4) return result; int size = num.size(); sort(num.begin(), num.end()); map<vector<int>, bool> m; for(int i = 0; i < size-3; i ++) { for(int j = i+1; j < size-2; j ++) { int k = j+1; //k < size-1 int l = size-1; while(k < l) { int sum = num[i]+num[j]+num[k]+num[l]; if(sum == target) { vector<int> cur(4,0); cur[0] = num[i]; cur[1] = num[j]; cur[2] = num[k]; cur[3] = num[l]; if(m.find(cur) == m.end()) { result.push_back(cur); m[cur] = true; } k ++; l --; } else if(sum > target) l --; else k ++; } } } return result; } };
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原文地址：http://www.cnblogs.com/ganganloveu/p/4176281.html
