[C exercise] inverted string

example ：

Input ：I like beijing.

Output ：beijing. like I

Method ： We can reverse the whole string first , Then reverse each word inside .

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

void reverse_arr(char * start ,char *end)
{
	assert(start);
	assert(end);
	char tmp = *start;
	*start = *end;
	*end = '\0';
	if (strlen(start) > 1)
	{
		reverse_arr(start + 1, end - 1);
	};
	*end = tmp;
}

void reverse_words(char* arr)
{
	assert(arr);
	char* start = arr;
	char* end = arr;
	while (*end != '\0')//end Point to '\0' when , Stop in reverse order 
	{
		while (*end != ' ' && *end != '\0')// Find the space , And I promise not '\0'
		{
			end++;
		}
		// The reverse 
		reverse_arr(start, end - 1);//end Point to space , Spaces do not need to be in reverse order , therefore -1
		if (*end != '\0')//end No '\0', be start Point to the new word after the space 
			start = ++end;
	}
}

int main()
{
	char arr[20] = "I like beijing.";
	int end = strlen(arr);
	// Reverse the entire string 
	reverse_arr(arr,arr+end-1);
	// Single word in reverse order 
	reverse_words(arr);
	printf("%s", arr);
	return 0;
}