编写一个程序，输入一篇文章，该文章小于1000字符，从中指定字符串换成另外一个。
#include<stdio.h>
#include<string.h>
#define N 1000
int main(void)
{
  char s[N],rs[N],ns[N];
  int lena,lenb,lenc;
  printf("Please input a string:\n");
  gets(s);
  printf("\n");
  printf("Please input the replaced string:\n");
  gets(rs);
  printf("\n");
  printf("Please input the new string:\n");
  gets(ns);
  printf("\n");
  lena=strlen(s);
  lenb=strlen(rs);
  lenc=strlen(ns);
  for(i=0;i<lena;i++)
  
}
写到这里写不下去了...大佬们求解

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

void str_replace( const char* src, const char* oldstr, const char* newstr, char* dest )
{
    const size_t oldstr_len = strlen( oldstr );
    const size_t newstr_len = strlen( newstr );

    for( const char* p; p=strstr(src,oldstr), p; src=p+oldstr_len )
    {
        memcpy( dest, src, p-src );
        dest += p-src;

        memcpy( dest, newstr, newstr_len );
        dest += newstr_len;
    }
    strcpy( dest, src );
}

#include <assert.h>

int main( void )
{
    char dest[1001];

    str_replace( "ababa", "a", "xx", dest );
    assert( strcmp(dest,"xxbxxbxx")==0 );

    str_replace( "bababab", "a", "xx", dest );
    assert( strcmp(dest,"bxxbxxbxxb")==0 );

    str_replace( "bbb", "a", "xx", dest );
    assert( strcmp(dest,"bbb")==0 );

    str_replace( "a", "a", "xx", dest );
    assert( strcmp(dest,"xx")==0 );

    str_replace( "", "a", "xx", dest );
    assert( strcmp(dest,"")==0 );
}