The character 'a' to 'z' are encoded as 1 - 26. Given a string of digits, compute the number of valid decodings of the string. For example, both 'aa' and 'k' can be encoded as '11'. Hence num_valid_encodings('11') = 2.

      recursive 

#include <stdio.h>
#include <string.h>
int count_decode(char S[10],int n)
{
    int x1=S[n-1]-'0',x2=S[n-2]-'0',z;
    if(!n)
        return 0;
    else if(n==1)
    {
        return 1;
    }
    else
    {
        z=x2*10+x1;
      //  printf("z=%d ",z);
        if(z>26)
            return (count_decode(S,n-1));
        else if(n==2)
            return (count_decode(S,n-1) + count_decode(S,n-2)+1);
        else
            return (count_decode(S,n-1) + count_decode(S,n-2));
    }
}
int main()
{
    int i,j,n;
    char S[10];
    gets(S);
    n=strlen(S);
    printf("< %d >",count_decode(S,n));
return 0;
}