求助

#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
 
using namespace std;
 
#define LL long long
#define MAXN 11000000
#define IL inline
#define RE register
#define FOR(i,a,b) for(RE int i=(a); i<=(b); i++)
#define FOR2(i,a,b) for(RE int i=(a); i>=(b); i--)
#define reset(a,b) memset(a,b,sizeof(a))
 
IL int read()
{
    RE int x=0,f=1;
    char c=getchar();
    while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}
    while(c>='0'&&c<='9'){x=x*10+(c-'0');c=getchar();}
    return x*f;
}
 
int ans,l;
 
char s[MAXN];
 
int b[31];
 
int main()
{
    scanf("%s",s+1);
    RE int n=strlen(s+1);
    l=1;
    FOR(i,1,n)
    {   
        RE int x=s[i]-'A'+1;
        if(b[x]>=l)
        {
            l=b[x]+1;
        }
        ans=max(ans,i-l+1);
        b[x]=i;
    } 
    printf("%d\n",ans);
    return 0;
}