leetcode 15,18. 3Sum,4Sum | 15,18. 三数之和,四数之和（Java）

Bertha 。 2022-08-30 00:49 141阅读 0赞

## 15. 3Sum ##

[https://leetcode.com/problems/3sum/][https_leetcode.com_problems_3sum]  
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class Solution { 
        public List<List<Integer>> threeSum(int[] nums) { 
            Set<Integer> seen = new HashSet<>();
            Set<List<Integer>> resultSet = new HashSet<>();
            Arrays.sort(nums);
            for (int i = 0; i < nums.length - 1; i++) { 
                for (int j = i + 1; j < nums.length; j++) { 
                    int t = -nums[i] - nums[j];
                    if (seen.contains(t)) { 
                        List<Integer> list = new ArrayList<>();
                        list.add(nums[i]);
                        list.add(nums[j]);
                        list.add(t);
                        resultSet.add(list);
                    }
                }
                seen.add(nums[i]);
            }
            return new ArrayList<>(resultSet);
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NpbmF0XzQyNDgzMzQx_size_16_color_FFFFFF_t_70 1]

## 18. 4Sum ##

[https://leetcode.com/problems/4sum/][https_leetcode.com_problems_4sum]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NpbmF0XzQyNDgzMzQx_size_16_color_FFFFFF_t_70 2]

class Solution { 
        public List<List<Integer>> fourSum(int[] nums, int target) { 
            Set<Integer> seen = new HashSet<>();
            Set<List<Integer>> resultSet = new HashSet<>();
            Arrays.sort(nums);
            for (int i = 0; i < nums.length - 2; i++) { 
                for (int j = i + 1; j < nums.length - 1; j++) { 
                    for (int k = j + 1; k < nums.length; k++) { 
                        int t = target - nums[i] - nums[j] - nums[k];
                        if (seen.contains(t)) { 
                            List<Integer> list = new ArrayList<>();
                            list.add(nums[i]);
                            list.add(nums[j]);
                            list.add(nums[k]);
                            list.add(t);
                            resultSet.add(list);
                        }
                    }
                }
                seen.add(nums[i]);
            }
            return new ArrayList<>(resultSet);
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NpbmF0XzQyNDgzMzQx_size_16_color_FFFFFF_t_70 3]

[https_leetcode.com_problems_3sum]: https://leetcode-cn.com/problems/3sum/
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NpbmF0XzQyNDgzMzQx_size_16_color_FFFFFF_t_70]: /images/20220829/bfa80e7aaf80444a8a2af6bc027f17a7.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NpbmF0XzQyNDgzMzQx_size_16_color_FFFFFF_t_70 1]: /images/20220829/ac9567d32a8b49f0b79091d1e84ab052.png
[https_leetcode.com_problems_4sum]: https://leetcode.com/problems/4sum/
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NpbmF0XzQyNDgzMzQx_size_16_color_FFFFFF_t_70 2]: /images/20220829/0e4ffb2b0d7d41c19a8f9270fc818073.png
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