leetcode18 4sum

我的leetcode代码已放入github：https://github.com/gaohongbin/leetcode

题目：

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.

思路：

和3sum的思路差不多。这里只贴代码。

代码：

 public List<List<Integer>> fourSum(int[] nums, int target) {        List<List<Integer>> list= new ArrayList<List<Integer>>();        if(nums==null || nums.length<4)        return list;                quickSort(nums,0,nums.length-1);        int length=nums.length;        int first=0;        int two=1;        int three=2;        int four=3;        while(first<length-3){        two=first+1;        while(two<length-2){        three=two+1;        four=length-1;        while(three<four){        if(nums[three]+nums[four]>target-nums[first]-nums[two])        four--;        if(nums[three]+nums[four]<target-nums[first]-nums[two])        three++;        if(nums[three]+nums[four]==target-nums[first]-nums[two] && three<four){        List<Integer> listThree=new ArrayList<Integer>();        listThree.add(nums[first]);        listThree.add(nums[two]);        listThree.add(nums[three]);        listThree.add(nums[four]);        list.add(listThree);        three++;        while(three<four){        if(nums[three]==nums[three-1])        three++;        if(nums[three]!=nums[three-1])        break;        }                }                }        two++;    while( two<length-2 && nums[two-1]==nums[two] )    two++;        }        first++;while( first<length-3 && nums[first-1]==nums[first] )first++;                }        return list;    }  public static void quickSort(int[] nums,int low,int high){if(low>=high || nums==null)return ;int length=nums.length;int mid=nums[low];int i=low,j=high;while(i<j){while(nums[j]>mid && j>i)j--;nums[i]=nums[j];while(nums[i]<=mid && i<j)i++;nums[j]=nums[i];}nums[j]=mid;quickSort(nums,low,j-1);quickSort(nums,j+1,high);}