Sword finger offer 05 (implemented in C language)

describe
Please implement a function , Replace each space in a string with “%20”. for example , When the string is We Are Happy. The replaced string is We%20Are%20Happy.

char* replaceSpace(char* s ) {
    
    char a[30] = {
     0 };
	int i = 0;
	int length = strlen(s);
	while (length--)
	{
    
		if (*s != ' ')
		{
    
			a[i] = *s;
			++i;
			s++;
		}
		else
		{
    
			a[i] = '%';
			a[i + 1] = '2';
			a[i + 2] = '0';
			i += 3;
			s++;
		}
	}
	return a;
}