atoi()函数实现

实现atoi()函数，在出错时返回0；
 1 int atoi(const char *str)
 2 {
 3     if((str == NULL) || (strlen(str) == 0)) {
 4         return 0;
 5     }
 6 
 7     int value = 0;
 8     int sign = 0;
 9     int index = 0;
10     int minMark = 0;
11     const int MAX = 2147483647;
12     const int MIN = -2147483648;
13     while(str[index] != '\0') {
14         char c = str[index];
15         if((c == '+') || (c == '-')) {
16             if(sign == 0) {
17                 sign = (c == '+' ? 1 : -1);
18             }else {
19                 return 0;
20             }
21         }else if((c >= '0') && (c <= '9')) {
22             if(value > MAX / 10) {
23                 return 0;
24             }else if(value == MAX / 10) {
25                 if(c - '0' > MAX % 10 + 1) {
26                     return 0;
27                 }else if((c - '0' == MAX % 10 + 1) && (sign == -1) && (minMark == 0)) {
28                     minMark = 1;
29                     value = value * 10 + MAX % 10;
30                 }else if(c - '0' <= MAX % 10) {
31                     value = value * 10 + c - '0';
32                 }else {
33                     return 0;
34                 }
35             }else{
36                 value = value * 10 + c - '0';
37             }
38         }else {
39             return 0;
40         }
41         index ++;
42     }
43     if(minMark == 1) {
44         return MIN;
45     }else if(sign == -1) {
46         value *= -1;
47     }
48     return value;
49 }