LeetCode || 大杂烩w

454. 4Sum II

题意：给四个数组，每个数组内取一个数使得四个数和为0，问有多少种取法

思路：枚举为On4，考虑两个数组，On2枚举所有可能的和，将和的出现次数存入map中，On2枚举另两个数组，看是否加和为0

class Solution {
public:
    int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {
        int na = A.size(), nb = B.size(), nc = C.size(), nd = D.size();
        int cnt = 0;
        map<int, int> mp;
        for (int i = 0; i < na; i++) {
            for (int j = 0; j < nb; j++) {
                int sum = A[i] + B[j];
                if (mp[-sum]) mp[-sum]++;
                else mp[-sum] = 1;
            }
        }
        for (int i = 0; i < nc; i++) {
            for (int j = 0; j < nd; j++) {
                int sum = C[i] + D[j];
                if (mp[sum]) cnt += mp[sum];
            }
        }
        return cnt;
    }
};

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24. Swap Nodes in Pairs

题意：交换一个链表中每相邻两个元素

想看到就是多加个头的空指针操作（

ListNode *emptyhead = new ListNode(-1);
emptyhead -> next = head;

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode *emptyhead = new ListNode(-1);
        emptyhead -> next = head;
        ListNode *p = head, *q, *lst = emptyhead;
        while (lst -> next && lst -> next -> next) {
            p = lst -> next;
            q = p -> next;
            p -> next = q -> next;
            q -> next = p;
            lst -> next = q;
            lst = p;
        }
        return emptyhead -> next;
    }
};

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25. Reverse Nodes in k-Group

题意：将每k个元素倒序

Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5

思路：用两个指针来取倒序的一段

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        ListNode *emptyhead = new ListNode(-1);
        emptyhead -> next = head;
        ListNode *pre = emptyhead, *cur = emptyhead;
        int i = 0;
        while (cur -> next) {
            i++;
            cur = cur -> next;
            if (i % k == 0) {
                pre = reverse(pre, cur -> next);
                cur = pre;
            }
        }
        return emptyhead -> next;
    }
    ListNode* reverse(ListNode* pre, ListNode* nxt) {
        ListNode *lst, *cur;
        lst = pre -> next;
        cur = lst -> next;
        while (cur != nxt) {
            lst -> next = cur -> next;
            cur -> next = pre -> next;
            pre -> next = cur;
            cur = lst -> next;
        }
        return lst;
    }
    
};

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29. Divide Two Integers

题意：不用乘除模运算，完成整数乘法，若溢出则输出INT_MAX，向0取整

思路：思路是往下减，一个一个减布星，来移位

class Solution {
public:
    int divide(int dividend, int divisor) {
        long long res = 0;
        int flag = 1;
        if ((dividend < 0 && divisor > 0) || (dividend > 0 && divisor < 0)) {
            flag = -1;
        }
        long long m = abs((long long)dividend);
        long long n = abs((long long)divisor);
        long long base, t;
        while (m >= n) {
            base = n;
            t = 1;
            while (m >= (base << 1)) {
                base <<= 1;
                t <<= 1;
            }
            m -= base;
            res += t;
        }
        if (flag == -1) return int(-res);
        if (res > 2147483647) return 2147483647;
        return res;
    }
};

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31. Next Permutation

题意：给一个序列，输出字典序下一个的序列

思路：对于已经是字典序的序列特判，输出倒序；其他情况下，

从后往前找到第一个开始减小的元素

1 2 5 4 3 1

然后从后往前找到第一个比2大的元素，交换它们

1 3 5 4 2 1

然后把3之后的序列倒置

1 3 1 2 4 5

嘛拿纸笔画一画就好惹（

class Solution {
public:
    void nextPermutation(vector<int>& nums) {
        int n = nums.size();
        int flag = 0;
        int last, idx;
        for (int i = n - 2; i >= 0; i--) {
            if (nums[i] < nums[i + 1]) {
                last = nums[i];
                idx = i;
                flag = 1;
                break;
            }
        }
        if (flag == 0) {
            reverse(nums.begin(), nums.end());
        } else {
            for (int i = n - 1; i >= idx + 1; i--) {
                if (nums[i] > last) {
                    nums[idx] = nums[i];
                    nums[i] = last;
                    break;
                }
            }
            reverse(nums.begin() + idx + 1, nums.end());
        }
        return;
    }
};
// 1 2 4 5 6 -> 1 2 4 6 5
// 1 2 4 6 5 -> 1 2 5 4 6
// 1 3 4 2 -> 1 4 2 3
// 3 4 2 1 -> 4 1 2 3

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46. Permutations

题意：输出全排列

next_permutation 输出从当前排列之后的所有全排列（所以要 sort 和 do while）

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        vector<vector<int>> res;
        sort(nums.begin(), nums.end());
        do {
            res.push_back(nums);
        } while (next_permutation(nums.begin(), nums.end()));
        return res;
    }
};

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56. Merge Intervals

结构体vector的排序姿势……？

为啥cmp不行啊qwq（

struct Interval {
     int start;
     int end;
     Interval() : start(0), end(0) {}
     Interval(int s, int e) : start(s), end(e) {}
 };

vector<Interval>& intervals;

sort(intervals.begin(), intervals.end(), [](Interval &a, Interval &b) {return a.start < b.start;});