C语言 字符串相减

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

int main()
{
    char a[20],b[20];
    int i,j,k;
    unsigned long m,n;
    scanf("%s",a);
    scanf("%s",b);
    m=strlen(a);
    n=strlen(b);
    for (i=0; i<=n; i++) {
        for (j=0,k=0; j<=m; j++) {
            if (a[j]!=b[i]) {
                a[k++]=a[j];
            }
        }
    }
    //a[k]='\0';
    printf("%s",a);
    
    return 0;
}

只能应付一部分用例，当被减数有重复出现时会输出错误

abvjanv v

abjanvv

Program ended with exit code: 0