15.3sum.java

        class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> threeSumList = new ArrayList<List<Integer>>();
        if (nums.length<3) return threeSumList;
        Arrays.sort(nums);
       int start;
    
            for (int i= 0; i<nums.length-2;i ++){
                start = i+1;
               int  end  = nums.length-1;
                    while (start<end){
                    int sum = nums[start]+nums [end];
                    if (sum + nums[i]==0){
                   List<Integer>numsList =new ArrayList<Integer>();
                        numsList.add(nums[start]);
                        numsList.add(nums[end]);
                        numsList.add(nums[i]);
                        if(!threeSumList.contains(numsList)){
                                threeSumList.add(numsList);

                        }
                        start ++;
                        end --;
                    }
                    else if(sum+nums[i]<0){
                        start ++;
                    }
                        else {
                             end --;
                        }
                   
            }
            
        }
            return threeSumList;
    }
}