still it doesn't prevent the edge case when nums = [1,1,1,1], target = 4

KSumWithBug.java

class Solution {
    
    private List<List<Integer>> result = new ArrayList();
    public List<List<Integer>> fourSum(int[] nums, int target) {
        Arrays.sort(nums);
        List<Integer> previousNum = new ArrayList();
        kSum(previousNum, nums, 4, 0, target);
        return result;
    }
    
    public void kSum(List<Integer> previousNum, int[] nums, int k, int start, int target) {
        if(k > 2) {
            for(int i = start; i < nums.length-k+1; i++){
                if(i > start && nums[i] == nums[i-1]){
                    continue;
                }
                previousNum.add(nums[i]);
                kSum(previousNum, nums, k-1, i+1, target - nums[i]);
                previousNum.remove(previousNum.size()-1);
            }
        } else {
            int left = start;
            int right = nums.length -1;
            while(left < right){
            
                int sum = nums[left] + nums[right];
                if(sum < target){
                    left++;
                } else if(sum > target){
                    right--;
                } else {
                    List<Integer> subset = new ArrayList();
                    subset.addAll(previousNum);
                    subset.add(nums[left]);
                    subset.add(nums[right]);
                    result.add(subset);
                    left++;
                    while(left < right && nums[left] == nums[left-1] ){
                        left++;
                    }
                }
            }
        }
    }
    
    public int sumList(List<Integer> previousNum){
        int sum = 0;
        if(previousNum != null && !previousNum.isEmpty()){
            for(int i = 0; i < previousNum.size(); i++){
                sum += previousNum.get(i);
            }
        }
        return sum;
    }
}

// -2,-1,0,0,1,2