这道题和3Sum那道题十分类似，只是把3sum 中的0换成了target-num【i】，直接上代码。
public List<List<Integer>> fourSum(int[] num, int target) { List<List<Integer>> array=new ArrayList(); if(num == null||num.length < 4) return array; Arrays.sort(num); List<Integer> list=new ArrayList(); for(int i=0;i<=num.length-4;i++) { if(i>0&&num[i] == num[i-1]) continue; int begin=i+1; int threesum=target-num[i]; for(int j=begin;j<=num.length-3;j++) { if(j>begin&&num[j] == num[j-1]) continue; int left=j+1; int right=num.length-1; while(left<right) { int ans=num[j]+num[left]+num[right]; if(ans == threesum) { list.add(num[i]); list.add(num[j]); list.add(num[left]); list.add(num[right]); array.add(list); list=new ArrayList(); left++; while(left<right&&num[left] == num[left-1]) left++; right--; while(left<right&&num[right] == num[right+1]) right--; }else if(ans<threesum) left++; else right--; } } } return array; }