LeetCode 8. String to Integer (atoi)

Description

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

Implement atoi which converts a string to an integer.

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.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.

If no valid conversion could be performed, a zero value is returned.

Note:

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

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.

Explanation

The problem requires to write our own c++ atoi like function to parse the integer from the input string to an integer.

We can solve the problem by handing the following cases:

  • string with white spaces e.g. ”   1 23   “
  • string with a positive or negative sign e.g. “+123”
  • string with non-numerical characters e.g. “123@%^123”
  • string contains value out of integer range e.g. “99998888777766665555444433”

Java Solution

public class Solution {
    public int myAtoi(String str) {
        // remove whitespaces
        str = str.trim();
        
        if (str == null || str.length() == 0) {
            return 0;
        }
        
        double result = 0;
        
        // handle positive and negative sign
        boolean isNegative = false;
        int startIndex = 0; // index of first numerical character
        
        if (str.charAt(0) == '+' || str.charAt(0) == '-') {
            startIndex++;
        }
        if (str.charAt(0) == '-') {
            isNegative = true;
        }
        
        // handle normal case "123"
        for (int i = startIndex; i < str.length(); i++) {
            // handle non-numerical character
            if (str.charAt(i) < '0' || str.charAt(i) > '9') {
                break;
            }
            int digitValue = (int)(str.charAt(i) - '0'); // '9' - '0' 
            result = result * 10 + digitValue;
        }
        
        if (isNegative) {
            result = -result;
        }
        
        // handle out of Integer range
        if (result > Integer.MAX_VALUE) {
            return Integer.MAX_VALUE;
        }
        if (result < Integer.MIN_VALUE) {
            return Integer.MIN_VALUE;
        }

        return (int)result;
    }
}

Python Solution

class Solution:
    def myAtoi(self, str: str) -> int:
        str = str.strip()
                
        result = 0
                
        if len(str) < 1 or str is None:
            return result

        is_positive = True
        start_index = 0

        if str[0] == '-':
            is_positive = False
            start_index = 1        
        elif str[0] == '+':
            start_index = 1       

        if start_index >= len(str) or str[start_index].isdigit() is False:  
            return result            
            
        end_index = 0
        for i in range(start_index, len(str)):  
            if str[i].isdigit() is False:
                break
            end_index = i

                    
        result = float(str[start_index: end_index + 1])
        
        if not is_positive:
            result = -result
            
        if result > 2**31 - 1:
            return 2**31 - 1
        elif result < -2**31:
            return -2**31
        
        
        return int(result)

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