Bzoj 3662: Clever Rabbit

传送门：http://www.lydsy.com/JudgeOnline/problem.php?id=3662
题意：
聪明的兔子定义了三个函数:
• g(x) 表示将 x 十进制表示中各位数字从高位到低位降序排列组成的数字。
• L(x) 表示将 x 十进制表示中各位数字从高位到低位升序排列组成的数字。
• f(x)=g(x)−L(x)
如果一个数 x 与 f(x) 相等，兔子们称它为幸运数。现在兔子们希望计算在所有的 n 位数字中，所有幸运
数的平方的和。结果对 p 取模后输出，注意这里数字可以包含前导 0.
数据范围： 1≤n≤30 ;1≤p≤2×109
题解： 一看数据范围只有 30 就猜想是个打表题，30 个数 分成 10 类一共有 C930+9=211915132 种情况，再 O(n) 算一下感觉相当轻松嘛，然后就拿了个暴力打了个表（只用了7min多…），然后用python算了一下平方水过。
打表程序：

#include<bits/stdc++.h>const int N = 50;int n, t[N], a[N], b[N], c[N], d[N];void dfs(int dep, int last) {    if (!dep) {        for (int i = last + 1; i <= n; i++) t[i] = dep;        for (int i = 1; i <= n; i++)            a[i] = t[i], b[i] = t[n - i + 1];        for (int i = n; i > 0; i--) {            c[i] = a[i] - b[i];            if (c[i] < 0) c[i] += 10, a[i - 1]--;        }        for (int i = 1; i <= n; i++) d[i] = c[i];        std::sort(c + 1, c + n + 1);        bool suc = 1;        for (int i = 1; i <= n; i++)            if (c[i] != b[i]) suc = 0;        if (!d[1]) suc = 0;        if (suc) {            for (int i = 1; i <= n; i++)                printf("%d", d[i]);            printf("\n");        };        return;    }    for (int pos = last; pos <= n; pos++) {        for (int i = last + 1; i <= pos; i++) t[i] = dep;        dfs(dep - 1, pos);    }}int main() {    freopen("test.out", "w", stdout);    for (n = 3; n <= 30; n++) dfs(9, 0);    return 0;}

表：

#include<bits/stdc++.h>typedef long long ll;const char a[240][50] = 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char b[240][100] = 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main() {    int n, p, len;    scanf("%d%d", &n, &p);    ll ans = 0, t;    for (int i = 0; i < 240; i++)        if (strlen(a[i]) == n) {            t = 0; len = strlen(b[i]);            for (int j = 0; j < len; j++)                t = (t * 10 + b[i][j] - '0') % p;            ans += t;        }    printf("%lld\n", ans % p);    return 0;}