public List<List<Integer>> threeSum(int[] nums) {
    List<List<Integer>> result = new ArrayList<>();
    int len = nums.length;
    if (nums == null || len<3) {
        return result;
    }
    //sort是关键，排序完就很容易去重了
    Arrays.sort(nums);

    for (int i=0;i<len;i++){
        //nums[i] is the first integer of three
        // 去重
        //if (i>0 && nums[i] == nums[i-1]) continue;
        //用while一样效果，不用回到for去判断
        while (i>0 && i<len-1 && nums[i] == nums[i-1]){
            i++;
        }
        if (nums[i]>0) break;

        int left = i+1;
        int right = len-1;
        while (left<right){
            int sums = nums[i]+nums[left]+nums[right];
            if ( sums == 0){
                result.add(Arrays.asList(nums[i],nums[left],nums[right]));
                //必须是在找到答案后去重，不然会过滤掉正确答案
                // 去重
                while (left<right && nums[left]== nums[left+1]){
                    left++;
                }
                // 去重
                while (left<right && nums[right]== nums[right-1]){
                    right--;
                }
                left++;
                right--;
            }
            else if ( sums < 0){
                left++;
            }
            else{
                right--;
            }
        }
    }
    return result;
}