A leetcode.cn/problems/ant-on-the-boundary/description/" rel="nofollow">边界上的蚂蚁

模拟

class Solution {
public:
    int returnToBoundaryCount(vector<int> &nums) {
        int s = 0;
        int res = 0;
        for (auto x: nums) {
            s += x;
            if (s == 0)
                res++;
        }
        return res;
    }
};

---

B leetcode.cn/problems/minimum-time-to-revert-word-to-initial-state-i/description/" rel="nofollow">将单词恢复初始状态所需的最短时间 I

枚举：若经过 $i$ 秒后 $word$ 可以恢复到其初始状态，则说明 $word$ 长为 $n-i\times k$的后缀与长为 $n-i\times k$的前缀相等，$n$ 为 $word$ 的长度，升序枚举 $i$ 直到满足条件

class Solution {
public:
    int minimumTimeToInitialState(string word, int k) {
        int n = word.size();
        int i = 1;
        for (; i * k < n; i++) {
            int tag = 1;
            for (int j = i * k; j < n; j++)
                if (word[j] != word[j - i * k])
                    tag = 0;
            if (tag)
                return i;
        }
        return i;
    }
};

---

C leetcode.cn/problems/find-the-grid-of-region-average/description/" rel="nofollow">找出网格的区域平均强度

模拟：枚举网格中的 $3\times 3$ 的子网格，判断是否是区域，同时记录各位置所属于的区域数和所属于的区域的平均强度之和，最后求网格的区域平均强度

class Solution {
public:
    vector<vector<int>> resultGrid(vector<vector<int>> &image, int threshold) {
        int m = image.size(), n = image[0].size();
        vector<vector<int>> tot(m, vector<int>(n));
        vector<vector<int>> cnt(m, vector<int>(n));
        for (int i = 0; i + 2 < m; i++)
            for (int j = 0; j + 2 < n; j++) {
                int tag = 1;//判断是否是区域
                for (int x = i; x < i + 3; x++)
                    for (int y = j + 1; y < j + 3; y++)
                        if (abs(image[x][y] - image[x][y - 1]) > threshold)
                            tag = 0;
                for (int y = j; y < j + 3; y++)
                    for (int x = i + 1; x < i + 3; x++)
                        if (abs(image[x][y] - image[x - 1][y]) > threshold)
                            tag = 0;
                if (tag) {
                    int s = 0;
                    for (int x = i; x < i + 3; x++)
                        for (int y = j; y < j + 3; y++)
                            s += image[x][y];
                    s /= 9;//当前区域的平均强度
                    for (int x = i; x < i + 3; x++)
                        for (int y = j; y < j + 3; y++) {
                            cnt[x][y]++;//所属区域数目+1
                            tot[x][y] += s;//所属区域的平均强度之和+s
                        }
                }
            }
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                if (!cnt[i][j])
                    tot[i][j] = image[i][j];
                else
                    tot[i][j] /= cnt[i][j];
        return tot;
    }
};

---

D leetcode.cn/problems/minimum-time-to-revert-word-to-initial-state-ii/description/" rel="nofollow">将单词恢复初始状态所需的最短时间 II

字符串哈希 + 枚举：若经过 $i$ 秒后 $word$ 可以恢复到其初始状态，则说明 $word$ 长为 $n-i\times k$的后缀与长为 $n-i\times k$的前缀相等，$n$ 为 $word$ 的长度，升序枚举 $i$ 直到满足条件

class Solution {
public:
    int minimumTimeToInitialState(string word, int k) {
        int n = word.size();
        int i = 1;
        srand(time(0));
        shash h(word, 2333 + rand() % 100, 1e9 + rand() % 100);
        for (; i * k < n; i++) {     
            if (h(i * k, n - 1) == h(0, n - 1 - i * k))//s[i*k,n-1]和s[0,n-1-i*k]是否相等
                return i;
        }
        return i;
    }

    class shash {//字符串哈希模板
    public:
        using ll = long long;
        vector<ll> pres;
        vector<ll> epow;
        ll e, p;

        shash(string &s, ll e, ll p) {
            int n = s.size();
            this->e = e;
            this->p = p;
            pres = vector<ll>(n + 1);
            epow = vector<ll>(n + 1);
            epow[0] = 1;
            for (int i = 0; i < n; i++) {
                pres[i + 1] = (pres[i] * e + s[i]) % p;
                epow[i + 1] = (epow[i] * e) % p;
            }
        }

        ll operator()(int l, int r) {
            ll res = (pres[r + 1] - pres[l] * epow[r - l + 1] % p) % p;
            return (res + p) % p;
        }
    };
};