converts a string to an integer.(cpp,leetcode)

converts a string to an integer.

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The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.

Note:

1. Only the space character ' ' is considered as whitespace character.

1. Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−2³¹,  2³¹ − 1]. If the numerical value is out of the range of representable values, INT_MAX (2³¹ − 1) or INT_MIN (−2³¹) is returned.
   

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Example 1:

Input: "42"
Output: 42

Example 2:

Input: "   -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign.
             Then take as many numerical digits as possible, which gets 42.

Example 3:

Input: "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.

Example 4:

Input: "words and 987"
Output: 0
Explanation: The first non-whitespace character is 'w', which is not a numerical 
             digit or a +/- sign. Therefore no valid conversion could be performed.

Example 5:

Input: "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
             Thefore INT_MIN (−231) is returned.

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code:-

class Solution {
public:
    int myAtoi(string str)
    {
       
        int flage=0;
        while(str[0]==' ')
        {
            str.erase(0,1);
        }
        //////////////////////////
        if(str[0]=='-')
        {
            flage=1;
            str.erase(0,1);
        }
        else if(str[0]=='+')
        {
            flage=0;
            str.erase(0,1);
        }
        /////////////////////////
        
        if(isalpha(str[0]))
            return 0;
        
       //////////////////////////
       long int a=0;
        
       while(isdigit(str[0]))
       {
           a=a*10+((int)str[0]-48);
           str.erase(0,1);
           
           if(flage==1 && a>pow(2,31))
               return INT_MIN;
           if(flage==0 && a>=pow(2,31))
               return INT_MAX;
           
           
       }
        if(flage==1)
            return -a;
        else return a;
        
        
        
        
        
    }
};