poj1850

组合数学

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
using namespace std;

char st[15];

int com(int n, int r)
{
    if (n - r < r)
        r = n - r;
    int i, j, s = 1;
    for (i = 0, j = 1; i < r; ++i)
    {
        s *= (n - i);
        for (; j <= r && s % j == 0; ++j)
            s /= j;
    }
    return s;
}

int main()
{
    //freopen("t.txt", "r", stdin);
    gets(st);
    int len = strlen(st);
    if (len == 1)
    {
        printf("%d\n", st[0] - 'a' + 1);
        return 0;
    }
    bool ok = true;
    if (st[0] <= 'z' && st[0] >= 'a')
        ok = true;
    else
        ok = false;
    for (int i = 1; i < len; i++)
        if (!(st[i] <= 'z' && st[i] >= 'a' && st[i] > st[i - 1]))
            ok = false;
    if (!ok)
    {
        printf("0\n");
        return 0;
    }
    int ans = 26;
    for (int i = 2; i < len; i++)
        ans += com(26, i);
    ans += com(26, len) - com(26 - (st[0] - 'a'), len);
    for (int i = 1; i < len; i++)
        ans += com(26 - (st[i - 1] - 'a' + 1), len - i) - com(26 - (st[i] - 'a'
                ), len - i);
    printf("%d\n", ans + 1);
    return 0;
}