public class Solution {
public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
// the same as 3sum
ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
if(num==null|| num.length<4) return res;
Arrays.sort(num); // 为啥又忘
HashSet<ArrayList<Integer>> hs = new HashSet<ArrayList<Integer>>();
for(int i=0; i<num.length-3; i++){
for(int j=i+1; j<num.length-2; j++){
int low = j+1, high = num.length-1;
while(low<high){
int sum = num[i]+num[j]+num[low]+num[high];
if(sum==target){ // 看见兔子target，才下套 tmp
ArrayList<Integer> tmp = new ArrayList<Integer>();
tmp.add(num[i]);
tmp.add(num[j]);
tmp.add(num[low]);
tmp.add(num[high]);
if(!hs.contains(tmp)){
hs.add(tmp);
res.add(tmp);
}
low++;
high--;
}else if(sum<target){
low++;
}else
high--;
}
}
}
return res;
}
}
