#include <stddef.h>
#include <stdio.h>

unsigned int mystrlen(char *s){
    unsigned int i=0;
    if (s==NULL)
        return NULL;
    do {
        i++;
    } while (s[i]!=NULL);
    return i;
}

char *mystrchr(char *s, char c){
    unsigned int i=0;
    if (s==NULL)
        return NULL;
    while ((s[i]!=c) && (i<mystrlen(s)))
        i++;
    if (s[i]!=c)
        return NULL;
    return  s[i];
}

char *mystrcpy(char *dest, char *src){
    int i;
    if (dest==NULL || src==NULL)
        return NULL;
    for (i=0;i<=mystrlen(src);i++)
        dest[i]=src[i];
    return dest;
}

char *mystrcat(char *dest, char *src){
    unsigned int i,leng;
    leng=mystrlen(dest);
    if (dest==NULL || src==NULL)
        return NULL;
    for (i=0;i<mystrlen(src)+1;i++)
        dest[i+leng]=src[i];
    return dest;
}

int mystrcmp(char *s1, char *s2){
    int i=0;
    if (s1==NULL || s2==NULL)
        return NULL;
    do{
        if (s1[i]>s2[i])
            return 1;
        if (s1[i]<s2[i])
            return -1;
        i++;
    } while (mystrlen(s1)==i-1);
    if (i==mystrlen(s2))
        return 0;
    else
        return -1;
}