代码随想录第七天打卡|454.四数相加II， 383. 赎金信，15. 三数之和， 18. 四数之和

454.四数相加II

Python

class Solution:def fourSumCount(self, nums1: List[int], nums2: List[int], nums3: List[int], nums4: List[int]) -> int:n=len(nums1)res=0Map=defaultdict(int)for i in range(n):for j in range(n):Map[nums1[i]+nums2[j]]+=1for i in range(n):for j in range(n):if Map[-(nums3[i]+nums4[j])]>0:res+=Map[-(nums3[i]+nums4[j])]return res

C++

class Solution {
public:int fourSumCount(vector<int>& nums1, vector<int>& nums2, vector<int>& nums3, vector<int>& nums4) {int n=nums1.size();unordered_map<int,int> mp;for (int i=0;i<n;i++){for (int j=0;j<n;j++){mp[nums1[i]+nums2[j]]++;}}int res=0;for (int i=0;i<n;i++){for (int j=0;j<n;j++){if (mp[-(nums3[i]+nums4[j])]>0){res+=mp[-(nums3[i]+nums4[j])];}}}return res;}
};

383. 赎金信

C++

class Solution {
public:bool canConstruct(string ransomNote, string magazine) {unordered_map<char,int> mp;for (auto s:magazine)mp[s]++;for (char s:ransomNote){if (mp[s]>0){mp[s]--;}else return false;}    return true;}
};

Python

class Solution:def canConstruct(self, ransomNote: str, magazine: str) -> bool:Map=defaultdict(int)for s in magazine:Map[s]+=1for s in ransomNote:if Map[s]>0:Map[s]-=1else:return Falsereturn True

15. 三数之和

Python

class Solution:def threeSum(self, nums: List[int]) -> List[List[int]]:nums.sort()if nums[0]>0:return []res=[]for i in range(len(nums)-2):left=i+1right=len(nums)-1if i>0 and nums[i]==nums[i-1]:  continuewhile left<right:if nums[i]+nums[left]+nums[right]>0:right-=1elif nums[i]+nums[left]+nums[right]<0:left+=1else:res.append([nums[i],nums[left],nums[right]])while left<right and nums[right]==nums[right-1]:right-=1while left<right and nums[left]==nums[left+1]:left+=1left+=1right-=1return res

C++

class Solution {
public:vector<vector<int>> threeSum(vector<int>& nums) {vector<vector<int>> res;sort(nums.begin(),nums.end());if (nums[0]>0){return res;}int left=0,right=nums.size();for (int i=0;i<nums.size()-2;i++){if (i>0 && nums[i]==nums[i-1])continue;int left=i+1,right=nums.size()-1;while (left<right){if (nums[i]+nums[left]+nums[right]>0)right--;else if (nums[i]+nums[left]+nums[right]<0)left++;else {res.push_back(vector<int>{nums[i],nums[left],nums[right]});while (left<right && nums[left]==nums[left+1]) left++;while (left<right && nums[right]==nums[right-1]) right--;left++;right--;}}}return res;}
};

18. 四数之和

Python

class Solution:def fourSum(self, nums: List[int], target: int) -> List[List[int]]:nums.sort()res=[]for i in range(len(nums)-3):if i>0 and nums[i]==nums[i-1]:continuefor j in range(i+1,len(nums)-2):if j-1>i and nums[j]==nums[j-1]:continueleft,right=j+1,len(nums)-1get=target-nums[i]-nums[j]while left<right:if nums[left]+nums[right]>get:right-=1elif nums[left]+nums[right]<get:left+=1else:res.append([nums[i],nums[j],nums[left],nums[right]])while left<right and nums[left]==nums[left+1]:left+=1while left<right and nums[right]==nums[right-1]:right-=1left+=1right-=1  return res

C++

class Solution {
public:vector<vector<int>> fourSum(vector<int>& nums, int target) {sort(nums.begin(),nums.end());vector<vector<int>>res;if (nums.size()<4)return res;for (int i=0;i<nums.size()-3;i++){if (i>0 && nums[i]==nums[i-1])continue;for (int j=i+1;j<nums.size()-2;j++){if (j>i+1 && nums[j]==nums[j-1])continue;int left=j+1,right=nums.size()-1;while (left<right){if ((long)nums[left]+nums[right]+nums[i]+nums[j]>target)right--;else if ((long)nums[left]+nums[right]+nums[i]+nums[j]<target)left++;else{res.push_back(vector<int>{nums[left],nums[right],nums[i],nums[j]});while (left<right && nums[left]==nums[left+1])left++;while (left<right && nums[right]==nums[right-1])right--;left++;right--;}}}}return res;}
};