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 };
