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 1 #include<iostream>
 2 using namespace std;
 3 int atoi1(char *s)
 4 {
 5 int sign=1,num=0;
 6     if(*s=='-')
 7         sign=-1;
 8     else if(*s=='+')
 9         sign=1;
10     s++;
11     while((*s)!='\0')
12     {
13         num=num*10+(*s-'0');
14         s++;
15     }   
16     return num*sign;   
17 }
18 
19 int main()
20 {
21     char *s="-123567890";   
22     cout<<atoi1(s);
23     system("pause");
24 }
25 
26 
27 下面是微软对该函数的实现
28 long atol(const char *nptr)
29 {
30         int c;              
31         long total;         
32         int sign;           
33 
34         
35         while ( isspace((int)(unsigned char)*nptr) )
36             ++nptr;//吸收空字符
37 
38         c = (int)(unsigned char)*nptr++;
39         sign = c;           
40         if (c == '-' || c == '+')
41             c = (int)(unsigned char)*nptr++;    
42 
43         total = 0;
44 
45         while (isdigit(c)) {
46             total = 10 * total + (c - '0');     
47             c = (int)(unsigned char)*nptr++;    
48         }
49 
50         if (sign == '-')
51             return -total;
52         else
53             return total;   
54 }