POJ1496 Word Index【全排列】

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

int C[27][27];
void ZuHe()
{
    C[0][0] = C[1][0] = C[1][1] = 1;
    for(int i = 2; i <= 26; ++i)
    {
        C[i][0] = C[i][i] = 1;
        for(int j = 1; j < i; ++j)
            C[i][j] = C[i-1][j] + C[i-1][j-1];
    }
}

char str[20];

int main()
{
    ZuHe();
    while(~scanf("%s",str))
    {
        int ans = 0;

        int len = strlen(str);
        bool falg = true;
        for(int i = 1; i < len; ++i)
        {
            if(str[i] <= str[i-1])
            {
                falg = false;
                break;
            }
        }
        if( !falg )
        {
            printf("0\n");
            continue;
        }

        int Time = len-1;
        while(Time)
        {
            ans += C[26][Time];
            Time--;
        }

        int ch,ch1;
        for(int i = 0; i < len; ++i)
        {
            ch = str[i] - 'a';
            if(i == 0)
                ch1 = 0;
            else
                ch1 = str[i-1] - 'a' + 1;
            while(ch > ch1)
            {
                ans += C[26-ch][len-1-i];
                ch--;
            }
        }
        ans++;
        printf("%d\n",ans);
    }

    return 0;
}