A leetcode.cn/problems/count-symmetric-integers/">统计对称整数的数目

枚举 $x$

class Solution {
public:
    int countSymmetricIntegers(int low, int high) {
        int res = 0;
        for (int i = low; i <= high; i++) {
            string s = to_string(i);
            if (s.size() & 1)
                continue;
            int s1 = 0, s2 = 0;
            for (int k = 0; k < s.size(); k++)
                if (k < s.size() / 2)
                    s1 += s[k] - '0';
                else
                    s2 += s[k] - '0';
            if (s1 == s2)
                res++;
        }
        return res;
    }
};

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B leetcode.cn/problems/minimum-operations-to-make-a-special-number/">生成特殊数字的最少操作

双指针：则若字符串操作完后为 $0$ ，设字符串长为 $n$ ，则需要$n$ 或 $n-1$ （字符串中含 0）操作使得字符串变为 $0$ ， 若字符串操作完后至少有两位数字，则其最后两位只能是 { 25 , 50 , 75 , 00 } \{25, 50, 75, 00\} {25,50,75,00} 其中之一，枚举可能的后两位，用双指针计算要得到当前枚举值的最少操作数

class Solution {
public:
    int minimumOperations(string num) {
        vector<string> tar{"25", "50", "75", "00"};
        int n = num.size();
        int res = num.find('0') == num.npos ? n : n - 1;
        for (auto &s: tar) {
            int i = s.size() - 1;
            int j = n - 1;
            int cur = 0;//得到当前枚举值的最少操作数
            for (; i >= 0 && j >= 0;) {
                if (s[i] == num[j]) {
                    i--;
                    j--;
                } else {
                    j--;
                    cur++;
                }
            }
            if (i < 0)
                res = min(res, cur);
        }
        return res;
    }
};

---

C leetcode.cn/problems/count-of-interesting-subarrays/">统计趣味子数组的数目

前缀和：设数组 $li$ 有：$l{i}_i=\{\begin{array}{cc}1& ,nums[i]\%mod=k\\ 0& ,nums[i]\%mod\ne k\end{array}$，设 $li$ 上的前缀和为 $p{s}_i=({\sum }_{j=0}^{j<i}l{i}_i)\%mod$ ，设子数组 $nums[l,r]$ 为趣味子数组，则有：$(p{s}_{r+1}-p{s}_l)\%mod=k$，即有 $p{s}_l=((p{s}_{r+1}-k)\%mod+mod)\%mod$。

class Solution {
public:
    using ll = long long;

    long long countInterestingSubarrays(vector<int> &nums, int modulo, int k) {
        unordered_map<int, ll> cnt;//cnt[val]: 前缀和val出现的次数
        cnt[0] = 1;//前缀为空
        int s = 0;//当前前缀和
        ll res = 0;
        for (int i = 0; i < nums.size(); i++) {
            if (nums[i] % modulo == k)
                s = (s + 1) % modulo;
            int s_l = ((s - k) % modulo + modulo) % modulo;
            res += cnt[s_l];
            cnt[s]++;
        }
        return res;
    }
};

---

D leetcode.cn/problems/minimum-edge-weight-equilibrium-queries-in-a-tree/">边权重均等查询

倍增+枚举：1）预处理：设 $0$ 为树的根节点，枚举边的权重 $w\_id$，从树根开始 $dfs$ ，计算各节点 $u$ 到树根的路径上的边数 $level[u]$，以及节点 $u$ 到树根的路径上边权重为 $w\_id$ 的边的数目 $s[u][w\_id]$，求倍增数组 $p$： $p[u][j]$为与 $u$ 距离为 $2^j$的祖先节点。2）对一个查询 $(a,b)$，用倍增的方式求 $a$ 和 $b$ 的最近公共祖先 $c$ ，然后枚举 $w\_id$ ，将 $a$ 和 $b$ 间路径上的边的边权统一为 $w\_id$ 的操作数为：$$\left(level[a]-level[c]-(s[a][w\_id]-s[c][w\_id])\right)+\left(level[b]-level[c]-(s[b][w\_id]-s[c][w\_id])\right)$$

class Solution {
public:
    vector<int> minOperationsQueries(int n, vector<vector<int>> &edges, vector<vector<int>> &queries) {
        vector<pair<int, int>> e[n];//邻接表
        int mx_w = 0, mn_w = INT32_MAX;//最大权重、最小权重
        for (auto &ei: edges) {
            e[ei[0]].emplace_back(ei[1], ei[2]);
            e[ei[1]].emplace_back(ei[0], ei[2]);
            mx_w = max(mx_w, ei[2]);
            mn_w = min(mn_w, ei[2]);
        }
        int level[n], s[n][27];
        int p[n][15];
        function<void(int, int, int, int, int)> dfs = [&](int cur, int par, int lev, int sum, int w_id) {
            if (w_id == mn_w)//倍增数组一轮dfs即可计算
                for (int i = 0; i < 15; i++)
                    p[cur][i] = i != 0 ? p[p[cur][i - 1]][i - 1] : par;
            level[cur] = lev;
            s[cur][w_id] = sum;
            for (auto &[j, w]: e[cur])
                if (j != par)
                    dfs(j, cur, lev + 1, w == w_id ? sum + 1 : sum, w_id);

        };
        for (int i = mn_w; i <= mx_w; i++)//枚举w_id
            dfs(0, 0, 0, 0, i);

        vector<int> res;
        res.reserve(queries.size());
        for (auto &qi: queries) {
            int a = qi[0], b = qi[1];
            if (a == b) {
                res.push_back(0);
                continue;
            }
            if (level[a] < level[b])
                swap(a, b);
            int c = a;//c最终为a和b的最近公共祖先
            for (int step = level[a] - level[b], ind = 0; step >= (1 << ind); ind++)
                if (step >> ind & 1)
                    c = p[c][ind];
            if (c != b) {
                int b_ = b;
                for (int ind = 14; ind >= 0; ind--) {
                    if (p[c][ind] != p[b_][ind]) {
                        c = p[c][ind];
                        b_ = p[b_][ind];
                    }
                }
                c = p[c][0];
            }
            int res_i = INT32_MAX;
            for (int w_id = mn_w; w_id <= mx_w; w_id++) {//枚举w_id
                int t1 = level[a] - level[c] - (s[a][w_id] - s[c][w_id]);
                int t2 = level[b] - level[c] - (s[b][w_id] - s[c][w_id]);
                res_i = min(res_i, t1 + t2);
            }
            res.push_back(res_i);
        }
        return res;
    }
};