代码随想录算法训练营DAY7第三章 哈希表part02

目录

[454. 四数相加 II](#454. 四数相加 II)

[383. 赎金信](#383. 赎金信)

[15. 三数之和](#15. 三数之和)

[18. 四数之和](#18. 四数之和)

---

454. 四数相加 II

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

383. 赎金信

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

15. 三数之和

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

18. 四数之和

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