LeetCode 8. String to integer (ATOI) (medium, string)

Title Description ： Please come to realize a myAtoi(string s) function , Enable it to convert a string to a 32 Bit signed integer （ similar C/C++ Medium atoi function ）.

function myAtoi(string s) The algorithm is as follows ：

1.  Read in strings and discard useless leading spaces 
2.  Check the next character （ Suppose you haven't reached the end of the character yet ） Positive or negative , Read the character （ If there is ）.  Determine whether the final result is negative or positive .  If neither exists , Suppose the result is positive .
3.  Read in the next character , Until you reach the next non numeric character or the end of the input . The rest of the string will be ignored .
4.  Convert the numbers read in the previous steps into integers （ namely ,"123" -> 123, "0032" -> 32）. If you don't read in the numbers , Then the integer is  0 . Change the symbol if necessary （ From step  2  Start ）.
 If the number of integers exceeds  32  Bit signed integer range  [−231,  231 − 1] , You need to truncate this integer , Keep it in this range . say concretely , Less than  −231  The integer of should be fixed to  −231 , Greater than  231 − 1  The integer of should be fixed to  231 − 1 .
5.  Returns an integer as the final result .

Be careful ：

 The white space character in this question only includes the space character  ' ' .
 Except for the leading space or the rest of the string after the number , Do not ignore   Any other character .

Example 1：

 Input ：s = "42"
 Output ：42
 explain ： The bold string is the character that has been read in , The caret is the character currently read .
 The first  1  Step ："42"（ No characters are currently read in , Because there are no leading spaces ）
         ^
 The first  2  Step ："42"（ No characters are currently read in , Because it doesn't exist here  '-'  perhaps  '+'）
         ^
 The first  3  Step ："42"（ Read in  "42"）
           ^
 Parse to get an integer  42 .
 because  "42"  In scope  [-231, 231 - 1]  Inside , The final result is  42 .

Example 2：

 Input ：s = " -42"
 Output ：-42
 explain ：
 The first  1  Step ：" -42"（ Read in leading space , But ignore ）
            ^
 The first  2  Step ：" -42"（ Read in  '-'  character , So the result should be negative ）
             ^
 The first  3  Step ：" -42"（ Read in  "42"）
               ^
 Parse to get an integer  -42 .
 because  "-42"  In scope  [-231, 231 - 1]  Inside , The final result is  -42 .

Example 3：

 Input ：s = "4193 with words"
 Output ：4193
 explain ：
 The first  1  Step ："4193 with words"（ No characters are currently read in , Because there are no leading spaces ）
         ^
 The first  2  Step ："4193 with words"（ No characters are currently read in , Because it doesn't exist here  '-'  perhaps  '+'）
         ^
 The first  3  Step ："4193 with words"（ Read in  "4193"; Because the next character is not a number , So read in stop ）
             ^
 Parse to get an integer  4193 .
 because  "4193"  In scope  [-231, 231 - 1]  Inside , The final result is  4193 .

Tips ：

0 <= s.length <= 200
s  By the English letters （ Uppercase and lowercase ）、 Numbers （0-9）、' '、'+'、'-'  and  '.'  form 

Code implementation ：

    public static int myAtoi(String s){
    
        if(s.length() == 0){
    
            return 0;
        }

        int index = 0; //  Traverse to the current character position 
        long res = 0;  //  final result 
        int sign = 1;  // 1 Is an integer , -1 It's a negative number 
        int length = s.length();  //  The length of the array 

        //  Remove spaces logically 
        while (index < length && s.charAt(index) == ' '){
    
            index++;
        }

        if(index == length){
    
            return 0;
        }

        //  Judging symbols 
        if(s.charAt(index) == '-' || s.charAt(index) == '+'){
    
            sign = s.charAt(index++) == '+' ? 1 : -1; 
        }

        for (; index < length; index++) {
    
            int digit = s.charAt(index) - '0';
            if(digit < 0 || digit > 9){
      //  If it is not a number, exit directly 
                break;
            }
            res = res * 10 + digit; 
            //  If the maximum value is exceeded , Return the maximum value directly 
            if(res * sign > Integer.MAX_VALUE){
    
                return Integer.MAX_VALUE;
            }
            //  If it's less than the minimum , Return the minimum value directly 
            if(res * sign < Integer.MIN_VALUE){
    
                return Integer.MIN_VALUE;
            }
        }
        return (int) (sign * res);
    }

source ： Power button （LeetCode）
link ：https://leetcode.cn/problems/string-to-integer-atoi
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