凯撒密码：c语言（麻烦大神帮我看看哪里出错了）

#include <stdio.h>
#include <string.h>

int main()
{
    int n,i,count;
    char a[100000];
    scanf("%d",&n);
    getchar();
    gets(a);
    count = strlen(a);
    for(i = 0; i < count; i ++)
    {
        if(('a'<=a[i]&&a[i]<='z')||('A'<=a[i]&&a[i]<='Z'))
        {
            if((('a'<=a[i])&&(a[i]<='z')==1)&&((('a'<=(a[i]+n))&&((a[i]+n)<='z'))==0))
                a[i]=a[i]-26+n;
            else if((('A'<=a[i])&&(a[i]<='Z')==1)&&((('A'<=(a[i]+n))&&((a[i]+n)<='Z'))==0))
               a[i]=a[i]+n-26;
                else a[i]=a[i]+n;
        }
    }
    printf("%s",a);
    return 0;
}

一直找不到错误在哪，新手一只，可能写得不是很清晰
结果显示我部分通过部分错误
麻烦大家帮帮忙！~（初来乍到，没有悬赏分给大家，拜托了~

https://www.cnblogs.com/ccccnzb/p/4008688.html

#include <stdio.h>
#include <string.h>

int main()
{
    int n, i, count, len;
    char a[100000];
    scanf("%d", &n);
    getchar();
    gets(a);
    count = strlen(a);
    for (i = 0; i < count; i++)
    {
        if (('a' <= a[i] && a[i] <= 'z'))
        {
            len = (n + a[i] - 'a') % 26;
            a[i] = len + 'a';
        }
    }
    printf("%s", a);
    return 0;
}