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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:
- 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)
算法1:排序后,固定前两个数,后两个数适用双指针,O(N^3).
java:
public class Solution { public List<List<Integer>> fourSum(int[] num, int target) { int len=num.length; Arrays.sort(num); List<List<Integer>> list =new LinkedList<List<Integer>>(); int i=0; while(i<len-3){ int j=i+1; while(j<len-2){ int k=j+1; int l=len-1; while(k<l){ int sum = num[i]+num[j]+num[k]+num[l]; if(sum==target){ List<Integer> lst = new LinkedList<Integer>(); lst.add(num[i]); lst.add(num[j]); lst.add(num[k]); lst.add(num[l]); list.add(lst); while(k+1<len-1&&num[k]==num[k+1]){ k++; } k++; while(l-1>2&&num[l]==num[l-1]){ l--; } l--; }else if(sum<target){ k++; }else{ l--; } } while(j+1<len-2&&num[j]==num[j+1]) j++; j++; } while(i+1<len-3&&num[i]==num[i+1]) i++; i++; } return list; } }
4Sum
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