算法记录Day 7 | 哈希表part02

算法记录Day 7 | 哈希表part02

LeetCode 454-四数相加II

题目链接：leetcode.cn/problems/4s…

题解

class Solution {
   public:
    int fourSumCount(vector<int> &nums1, vector<int> &nums2, vector<int> &nums3,
                     vector<int> &nums4) {
        unordered_map<int, int> map1;
        for (auto e1 : nums1) {
            for (auto e2 : nums2) {
                int sum = e1 + e2;
                map1[sum]++;
            }
        }
        int count = 0;
        for (auto e3 : nums3) {
            for (auto e4 : nums4) {
                int target = -e3 - e4;
                auto iter = map1.find(target);
                if (iter != map1.end()) {
                    count += iter->second;
                }
            }
        }
        return count;
    }
};

LeetCode 383-赎金信

题目链接：leetcode.cn/problems/in…

题解

class Solution {
   public:
    bool canConstruct(string ransomNote, string magazine) {
        unordered_map<char, int> map;
        for (char c : magazine) {
            map[c]++;
        }
        for (char c : ransomNote) {
            auto iter = map.find(c);
            if (iter != map.end()) {
                if (iter->second < 1) {
                    return false;
                } else {
                    map[c]--;
                }
            } else {
                return false;
            }
        }
        return true;
    }
};

LeetCode 15-三数之和

题目链接：leetcode.cn/problems/3s…

题解

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

LeetCode 18-四数之和

题目链接：leetcode.cn/problems/tw…

题解

class Solution {
   public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        if (nums.size() < 4) {
            return vector<vector<int>>();
        }
        sort(nums.begin(), nums.end());
        vector<vector<int>> res;
        for (int i = 0; i < nums.size() - 3; i++) {
            long n1 = nums[i];
            if (i > 0 && n1 == nums[i - 1]) {
                continue;
            }
            for (int j = i + 1; j < nums.size() - 2; j++) {
                long n2 = nums[j];
                if (j > i + 1 && n2 == nums[j - 1]) {
                    continue;
                }
                int l = j + 1, r = nums.size() - 1;
                while (l < r) {
                    long n3 = nums[l], n4 = nums[r];
                    if (n1 + n2 + n3 + n4 == target) {
                        res.push_back({nums[i], nums[j], nums[l], nums[r]});
                        while (l < r && n3 == nums[l]) {
                            l++;
                        }
                        while (l < r && n4 == nums[r]) {
                            r--;
                        }
                    } else if (n1 + n2 + n3 + n4 < target) {
                        l++;
                    } else {
                        r--;
                    }
                }
            }
        }
        return res;
    }
};