Convert String to Long

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：

 /*
  * Author: Min Li
  * Discussion:
  *    1. Return 0 when input is not valid (Empty or wrong format)
  *    2. Return LONG_MAX when input string overflows
  *    3. Return LONG_MIN when input string underflows
  *    4. Input String is allowed to have additional character after a valid substring that can form a long number. (e.g. +123+)
  *    5. Input can have many whitespaces before the first non-whitespace character. (e.g. "    123")
  *
  */

 // Class: Solution

 class Solution {
 public:
     // Method: Convert String to Long
     long stringToLong(string s) {
         // Special Case: Empty
         if(s.size()==)
           return ;
         long sign;                                          // Record the sign of the long number
         int index=;
         int strLen = s.size();                              // The length of input
         long result=;                                      // The final result
         // Discard the whitespaces before the first non-whitespace.
         while(index<strLen && s[index]==' ') index++;

         // The input only contains whitespaces
         if(index==strLen)
           return ;

         // Determine the sign
         if(s[index]=='-') {                                 // Input starts with "-"
           sign = -;
           ++index;
         }
         else if(s[index]=='+') {                            // Input starts with "+"
           sign = ;
           ++index;
         }
         else if(s[index] < '' || s[index] > '')           // Invalid input
           return ;
         else                                                // Unsigned input
           sign = ;

         // Retrieve the digit after first non-whitespace character
         while(index<strLen) {
             if(s[index]>='' && s[index]<='') {                                          // New character is 0-9
                 int digit = s[index]-'';
                 if(result>LONG_MAX/ || (result==LONG_MAX/ && digit>LONG_MAX%)) {      // Overflow or underflow
                     result = sign==-?LONG_MIN:LONG_MAX;
                 }
                 else
                    result = result*+digit;
             }
             else                                                                          // New character is not 0-9
                 break;
             index++;
         }

         return sign*result;

     }

     // Method: Test
     void test() {
         string testString;
 //        testString = "";                        // Test Case 1: Empty
         testString = "";                        // Test Case 2: Valid unsigned input
 //        testString = "+123";                    // Test Case 3: Valid signed input
 //        testString = "   123";                  // Test Case  : Valid input with whitespaces
 //        testString = "abc123";                  // Test Case  : Invalid input
 //        testString = "++123";                    // Test Case 4: Invalid signed input
 //        testString = "+123+";                    // Test Case 5: Valid input format with additional characters
 //        testString = "3924x8";                    // Test Case 6: Invalid input format
 //        testString = "1000000000000000000000";         // Test Case 7: Overflow
 //        testString = "-10000000000000000000";          // Test Case 8: Underflow
      cout << stringToLong(testString) << endl;
     }
 };