Leetcode:string to Integer (atoi)

1. Title

String to Integer (atoi) (string-to-number conversion)

2. Address of the topic

https://leetcode.com/problems/string-to-integer-atoi/

3. Topic content

English: Implement atoi to convert a string to an integer.

English: Implement the Atoi function to convert the input string (string type) to Integer data (integer type)

Tip: The Atoi function you implement needs to meet the following characteristics

• ignores all spaces (whitespace) before the first non-whitespace character of the string, the first non-whitespace character can be a sign, followed by a pure number, and the Atoi function converts these pure numbers to integers and returns

• Any other character can be followed by a numeric character, but these non-numeric characters are ignored

• If the first non-whitespace character in a string is not a number, the string is empty or consists only of spaces, it is not converted

• Returns 0 if the string cannot be converted. Returns the maximum value of the integer number Int_max (2147483647) or the minimum value int_min (-2147483648) If the value is too large or too small
  

4. Method of solving Problems 1

You can use the Integer.parseint function in Java to solve the problem, but the Integer.parseint function and the atoi described in the topic are more restrictive on the input being received, so you need to convert the input string first to the form that Integer.parseint can convert (a pure number And the preceding sign is extracted first, and then called Integer.parseint). In addition, in the process of numerical conversion, it is necessary to judge the exception information, if it is due to overflow caused by the numberformatexception, the integer number is returned to the maximum or minimum value, otherwise, can not be converted processing, directly return 0.

The Java code is as follows:

/** *  function Description:leetcode 8 - string to integer  (atoi)  *  Developer:tsybius2014 *  Development time: July 31, 2015  */public class Solution {         /**     * atoi function      *   @param  str     *  @return      */     public int myatoi (STRING&NBSP;STR)  {                 //input is empty direct return 0         if  (Str.isempty ())  {             return 0;        }                 str = str.trim ();            &nbSp;    char[] array = str.tochararray ();                 //judging the positive and negative       of numbers    boolean sign = true;        if  ( array[0] ==  '-')  {            sign  = false;        }                 //the first occurrence of the letter, ignore the following characters          int index = -1;        int signcount  = 0;        for (int i = 0; i <  array.length; i++)  {            if   (array[i] >=  ' 0 '  && array[i] <=  ' 9 ')  {                 continue;             } else if  (array[i] ==  '-'  | |  array[i] ==  ' + ')  {                 signcount++;                 //sign the second occurrence, ignore the following characters                  if  (signcount > 1)  {                     index = i;                      break;                }                 continue;             } else {                 index = i;                 break;             }        }                 //Intercept String         if   (index != -1)  {             Str = str.substring (0, index);             //after interception is empty string or invalid string, return 0&NBsp;           if  (Str.isEmpty ()  | |  str.equals ("-")  | |  str.equals ("+"))  {                 return 0;            }         }        //Value Conversion          int result;        try         {             result = integer.parseint (str);             return  (int) result;         }         catch  (NUMBERFORMATEXCEPTION&NBSP;EX)  //value overflow condition          {            if  (sign )  {                return  integer.max_value;            } else  {                return  Integer.MIN_VALUE;            }         }        catch  (Exception &NBSP;EX)  //other anomalies         {             return 0;        }     }}

5. method of solving problems 2

Method 2 is another form of implementation of Method 1, because Method 1 uses a comparison of the original method to parse the string, so I think I can use regular expressions directly match this number, to the Integer.parseint function.

The Java code is as follows:

import java.util.regex.pattern;import java.util.regex.matcher;/** *  function Description: LeetCode 8  - String to Integer  (atoi)  *  developer:tsybius2014 *  Development Date: July 31, 2015  */public class Solution {        /**      * atoi functions      *  @param  str      *  @return      */    public int myatoi (String  STR)  {                 str = str.trim ();         //regular Match          pattern p = pattern.compile ("^ ([+-]|) [0-9]+ ");           matcher m =  P.matcher (str);            if  (M.find ())  {             str = str.substring (M.start (),  m.end ());         } else {            return 0;         }                 //judgment number of the sign         boolean  sign = true;        if  (str.charAt (0)  ==  '-')  {            sign = false;         }                 //Value Conversion         int result;         try        {             result = integer.parseint (str);             return  (int) result;          }        catch  (Numberformatexception ex)  // Case of a value overflow         {             if  (sign)  {                 return Integer.MAX_VALUE;             } else {                 return Integer.MIN_VALUE;             }        }        catch  ( EXCEPTION EX)  //other anomalies         {             return 0;        }     }}

6. method of solving problems 3

If the above two methods are relatively lazy approach, then the third method can be called "normal solution", pure manual parsing string. It is important to note that, during the calculation, the result needs to be temporarily in a long type of data, which can be used to determine if there is an integer overflow in the same situation and return the correct return value (Integer.max_value or Integer.min_ VALUE).

The Java code is as follows:

/** *  function Description:leetcode 8 - string to integer  (atoi)  *  Developer:tsybius2014 *  Development time: July 31, 2015  */public class Solution {         /**     *  self-made atoi function      *   @param  str  string to convert      *  @return   converted numbers       */    public int myatoi (STRING&NBSP;STR)  {         str = str.trim ();         int  sign = 0; //symbol: 0 unsigned, 1 positive, 2 negative         long result  = 0; //Calculation Results                  for  (Char ch : str.tochararray ())  {             if  (ch ==  ' + ')  {                  //first encounter symbol record, second encounter symbol ignore                  if  (sign != 0)  {                      break;                 }                 sign = 1;             } else if  (ch ==  '- ')  {                  if  (sign != 0)  {                     break;                 }                 sign = -1;            } else  if  (ch >=  ' 0 '  && ch <=  ' 9 ')  {                  //encounters a number and checks if the maximum value of the integer is exceeded                 result *=  10;                 result +=  (ch -  ' 0 ');                 if  (Result > integer.max_value)  {                     if  (sign >= 0)  {                          return Integer.MAX_VALUE;                     } else {                          return Integer.MIN_VALUE;                     }                 }             } else {                  //encountered other characters directly exiting the loop                  break;            }         }                 if  (sign >= 0)  {             return  (int) result;        } else {             return  (int)-result;         }    }}

END

Leetcode:string to Integer (atoi)