Write a program to convert string to number without using library function。

1、问题

/*

*/

2、算法

#define MAX_LONG 0X7FFFFFFF

long foo(const char* str)

{

int sign = 1 ;

long num = 0 ;

const char* p = str ;

//假设输入的字符串是合法的

if ( *p == ‘-‘ )

{

sign = -1 ;

p++ ;

}else if ( *p == ‘+‘ )

{

sign = 1 ;

p++ ;

}

while(*p)

{

if ( (num > LONG_MAX/10) ||

(        (num == LONG_MAX/10) &&

( (-1 == sign && *p > ‘8‘) || (1 == sign && *p > ‘7‘)) ))

{

printf("out of bound") ;

return 0 ;

}

if ( *p >=‘0‘ && *p <=‘9‘ )

{

num = num*10 + (*p - ‘0‘) ;

}

*p++ ;

}

printf("%d", num * sign) ;

return num * sign ;

}