问题：

Given a string, write a routine that converts the string to a long, without using the built in functions that would do this. Describe what (if any) limitations the code has.

代码：

 1 /*
 2  * Author: Min Li
 3  * Discussion:
 4  *    1. Return 0 when input is not valid (Empty or wrong format)
 5  *    2. Return LONG_MAX when input string overflows
 6  *    3. Return LONG_MIN when input string underflows
 7  *    4. Input String is allowed to have additional character after a valid substring that can form a long number. (e.g. +123+)
 8  *    5. Input can have many whitespaces before the first non-whitespace character. (e.g. "    123")
 9  *
10  */
11
12
13 // Class: Solution
14
15 class Solution {
16 public:
17     // Method: Convert String to Long
18     long stringToLong(string s) {
19         // Special Case: Empty
20         if(s.size()==0)
21           return 0;
22         long sign;                                          // Record the sign of the long number
23         int index=0;
24         int strLen = s.size();                              // The length of input
25         long result=0;                                      // The final result
26         // Discard the whitespaces before the first non-whitespace.
27         while(index<strLen && s[index]==‘ ‘) index++;
28
29         // The input only contains whitespaces
30         if(index==strLen)
31           return 0;
32
33         // Determine the sign
34         if(s[index]==‘-‘) {                                 // Input starts with "-"
35           sign = -1;
36           ++index;
37         }
38         else if(s[index]==‘+‘) {                            // Input starts with "+"
39           sign = 1;
40           ++index;
41         }
42         else if(s[index] < ‘0‘ || s[index] > ‘9‘)           // Invalid input
43           return 0;
44         else                                                // Unsigned input
45           sign = 1;
46
47         // Retrieve the digit after first non-whitespace character
48         while(index<strLen) {
49             if(s[index]>=‘0‘ && s[index]<=‘9‘) {                                          // New character is 0-9
50                 int digit = s[index]-‘0‘;
51                 if(result>LONG_MAX/10 || (result==LONG_MAX/10 && digit>LONG_MAX%10)) {      // Overflow or underflow
52                     result = sign==-1?LONG_MIN:LONG_MAX;
53                 }
54                 else
55                    result = result*10+digit;
56             }
57             else                                                                          // New character is not 0-9
58                 break;
59             index++;
60         }
61
62
63         return sign*result;
64
65     }
66
67     // Method: Test
68     void test() {
69         string testString;
70 //        testString = "";                        // Test Case 1: Empty
71         testString = "123";                        // Test Case 2: Valid unsigned input
72 //        testString = "+123";                    // Test Case 3: Valid signed input
73 //        testString = "   123";                  // Test Case  : Valid input with whitespaces
74 //        testString = "abc123";                  // Test Case  : Invalid input
75 //        testString = "++123";                    // Test Case 4: Invalid signed input
76 //        testString = "+123+";                    // Test Case 5: Valid input format with additional characters
77 //        testString = "3924x8";                    // Test Case 6: Invalid input format
78 //        testString = "1000000000000000000000";         // Test Case 7: Overflow
79 //        testString = "-10000000000000000000";          // Test Case 8: Underflow
80      cout << stringToLong(testString) << endl;
81     }
82 };