Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target

Note:

The solution set must not contain duplicate quadruplets.

Example:

Given array nums = [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]
]

解答

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> outList = new ArrayList<List<Integer>>();
        int n = nums.length;
        if(n < 4) {
            return outList;
        }
        int t1,t2,sum,i,j,k,s;
        Arrays.sort(nums);
        t1 = nums[0];
        for(i=0; i<n-3; i++){    
	        if(i != 0 && t1 == nums[i]) continue;
	        t2 = nums[i+1];
	        for(j = i+1; j<n-2; j++){
	        	if(t2 == nums[j] && j>i+1) continue;
	        	sum = target - nums[i] -nums[j];
	        	k = j+1;
	        	s = n-1;
                int t3,t4;
	        	while(k<s){
                    t3 = nums[k];
	        		t4 = nums[s];
	        		if(nums[k] + nums[s] == sum){
	        			List<Integer> innerlist = new ArrayList<>();
	        			innerlist.add(nums[i]);
	        			innerlist.add(nums[j]);
	        			innerlist.add(nums[k]);
	        			innerlist.add(nums[s]);
	        			outList.add(innerlist);
	        			k++;
	        			s--;
                        while(t3 == nums[k] && k > j+1 && k<s) {k++;}
	        			while(t4 == nums[s] && s < n-1 && k<s) {s--;}
	        		}else if(nums[k] + nums[s] > sum){
	        			s--;
                        while(t4 == nums[s] && s < n-1 && k<s) {s--;}
	        		}else{
	        			k++;
                        while(t3 == nums[k] && k > j+1 && k<s) {k++;}
	        		}
	        		t2 = nums[j];
	        	}
	        	t1 = nums[i];
	        }
        }
        return outList;
    }
}