Reverse the characters in the parameter string

Write a function reverse_string(char * string) (recursive implementation)
implementation: reverse the characters in the parameter string.
Requirement: String manipulation functions in
the

Method 1: Recursion

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
int my_strlen(const char *p)
{
    return assert(p), !(*p) ? 0 : (1 + my_strlen(p + 1));
}
void reverse_string(char *p)
{
    if (*p == '\0')
        ;
    else
    {
        char tmp = p[0];
        int len = my_strlen(p);
        p[0] = p[len - 1];
        p[len - 1] = '\0';
        reverse_string(p + 1);
        p[len - 1] = tmp;
    }
}
int main()
{
    char arr[] = "abcdef";
    reverse_string(arr);
    printf("arr=%s \n",arr);
    system("pause");
    return 0;
}

Method 2: Non-recursive

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
void swap(char *left, char *right)
{
    assert(left != NULL);
    assert(right != NULL);
    while (left < right)
    {
        char tmp = *left;
        *left = *right;
        *right = tmp;
        left++;
        right--;
    }
}
int my_strlen(const char *p)
{
    return assert(p), !(*p) ? 0 : (1 + my_strlen(p + 1));
}
char *reverse_string(char * string)
{
    char *p = string;
    int len = my_strlen(string);
    swap(string, string + len - 1);
    return p;
}
int main()
{
    char arr[] = "abcdef";
    printf("%s\n", reverse_string(arr));
    system("pause");
    return 0;
}