c语言atoi与itoa用法以及超过long long int 型变量整数加减法

大整数的计算可以通过模运算来进行简化，也可以将其作为字符串读入，然后分段运算之后再进行拼接。

附：
atoi和itoa的示范代码如下：

#include <stdlib.h>
#include <stdio.h>

void main( void )
{
char *s; int ix;
char buffer[20];
int i = 3445;
long l = -344115L;
unsigned long ul = 1234567890UL;

s = " -9885 pigs"; /* Test of atoi */
ix = atoi( s );
printf( "atoi test: ASCII string: %s\t\tinteger: %d\n", s, ix );

_itoa( i, buffer, 10 );
printf( "String of integer %d (radix 10): %s\n", i, buffer );
_itoa( i, buffer, 16 );
printf( "String of integer %d (radix 16): 0x%s\n", i, buffer );
_itoa( i, buffer, 2 );
printf( "String of integer %d (radix 2): %s\n", i, buffer );

_ltoa( l, buffer, 16 );
printf( "String of long int %ld (radix 16): 0x%s\n", l,