Java回溯版（超時）

package leetcode;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class demo_18 {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> list=new ArrayList<List<Integer>>();
        Arrays.sort(nums);
        
 
long sum=0;
        backtrack(list, new ArrayList<Integer>(), sum, target, nums, 0);
        System.out.println(list);
        return list;
    }
    public void backtrack(List<List<Integer>> list,List<Integer> alist,long sum,int target,int nums[],int j) {
        if(sum==target&&alist.size()==4) {
            list.add(
 
new ArrayList<Integer>(alist));
        }
        else {
            for(int i=j;i<nums.length;i++) {
                if(i>j&&nums[i]==nums[i-1]) {continue;}
                j=i;
                if(alist.size()<4) {
                    sum=sum+nums[i];
                    alist.add(nums[i]);
                    backtrack(list, alist, sum, target, nums, j
 
+1);
                    alist.remove(alist.size()-1);
                    sum=sum-nums[i];
                }
                else {
                    break;
                }
            }
        }
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        demo_18 d18=new demo_18();
        int nums[]= {1,0,-1,0,-2,2,0,0,0};
        d18.fourSum(nums, 0);
    }

}