hash POJ 1200 Crazy Search

Crazy Search
Time Limit: 1000MS Memory Limit: 65536K
Total Submissions: 30502 Accepted: 8433
Description

Many people like to solve hard puzzles some of which may lead them to madness. One such puzzle could be finding a hidden prime number in a given text. Such number could be the number of different substrings of a given size that exist in the text. As you soon will discover, you really need the help of a computer and a good algorithm to solve such a puzzle.
Your task is to write a program that given the size, N, of the substring, the number of different characters that may occur in the text, NC, and the text itself, determines the number of different substrings of size N that appear in the text.

As an example, consider N=3, NC=4 and the text “daababac”. The different substrings of size 3 that can be found in this text are: “daa”; “aab”; “aba”; “bab”; “bac”. Therefore, the answer should be 5.
Input

The first line of input consists of two numbers, N and NC, separated by exactly one space. This is followed by the text where the search takes place. You may assume that the maximum number of substrings formed by the possible set of characters does not exceed 16 Millions.
Output

The program should output just an integer corresponding to the number of different substrings of size N found in the given text.
Sample Input

3 4
daababac
Sample Output

5
Hint

Huge input,scanf is recommended.
Source

Southwestern Europe 2002

思路：
一开始我用的map,错了，据说会超时，因为数据量太大了，所以得用hash，把每个字母标记一个特有的数字，根据字符串的m进制不同做key;
代码：

#include<cstdio>#include<algorithm>#include<iostream>#include<cstring>#include<cmath>#define N 16000003int num[N];int hash[N];using namespace std;int main(){    int n,m;    string s;    int cnt=0;    while(~scanf("%d%d",&n,&m))    {        cin>>s;        num[s[0]]=cnt++;        for(int i=1; i<s.size(); i++)        {            if(num[s[i]]==0)            {                num[s[i]]=cnt++;            }        }        int i,j;        int p=0;        for(i=0; i<=s.size()-n; i++)        {            int sum=0;            for(j=0; j<n; j++)            {                sum=sum*m+num[s[i+j]];            }            if(!hash[sum])            {                p++;                hash[sum]=true;            }        }        cout<<p<<endl;    }}