为什么这种读数方式无法通过

下面这段是AC代码

#include <iostream>
#include <cstring>
#define LEN 4100

void clear(int a[])
{
    for (int i = 0; i < LEN; ++i)
        a[i] = 0;
}

void read(int a[])
{
    char s[LEN + 1];
    scanf("%s", s);

    clear(a);

    int len = strlen(s);

    for (int i = 0; i < len; ++i)
        a[len - i - 1] = s[i] - '0';
}

void write(int a[])
{
    int i;
    for (i = LEN - 1; i >= 1; --i)
        if (a[i] != 0)
            break;
    for (; i >= 0; --i)
        printf("%d", a[i]);
}

void mul(int a[], int b[], int c[])
{
    clear(c);

    for (int i = 0; i < LEN - 1; ++i)
    {
        for (int j = 0; j <= i; ++j)
            c[i] += a[j] * b[i - j];

        if (c[i] >= 10)
        {
            c[i + 1] += c[i] / 10;
            c[i] %= 10;
        }
    }
}

int main()
{
    int a[LEN];
    int b[LEN];
    int c[LEN];

    read(a);
    read(b);

    mul(a, b, c);

    // write(a);
    // printf("\n");
    // write(b);
    // printf("\n");
    write(c);

    return 0;
}

但是如果把read函数改为

clear(a);

    int i = 0;
    int temp;
    char ch = getchar();

    while (ch >= '0' && ch <= '9')
    {
        a[i] = ch - '0';
        ++i;
        ch = getchar();
    }

    for (int j = 0; j < i / 2; j++)
    {
        temp = a[j];
        a[j] = a[i - j - 1];
        a[i - j - 1] = temp;
    }

后面几个就都会WA,并且都是第一行第一列为0,调了好久,不清楚是为什么,在本地输出结果也是对的,看不出来问题,求大佬帮助