把i<len - 1改成i<len就AC，为什么？

#include<cstdio>
#include<cstring>
#include<cctype>
int main(){
	char in[2000];
	int alpha[30] = {0} , len;
	for ( int i = 0 ; i < 4 ; i ++ ) {
		fgets(in,sizeof(in),stdin);
		len = strlen(in);
		for ( int i = 0 ; i < len - 1; i ++ ){
			if (!isalpha(in[i])) continue;
			alpha[in[i]-'A']++;
		}
	}
	int max = -1;
	for ( int i = 0 ; i < 26 ; i ++ )
		if ( alpha[i] > max) max = alpha[i];
	for ( int i = max ; i > 0 ; i -- ) {
		for ( int j = 0 ; j < 26 ; j++ ) {
			if (i > alpha[j]) printf("  ");
			else printf("* ");
		}
		printf("\n");
	}
	for ( int i = 0 ; i < 26 ; i ++ ) {
		printf("%c ",'A'+i);
	}
	return 0;
}