[LeetCode] 8. String to Integer (atoi)

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Problem

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: [−2^31, 2^31 − 1]. If the numerical value is out of the range of representable values, INT_MAX (2^31 − 1) or INT_MIN (−2^31) is returned.

Example 1:

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Input: "42"
Output: 42

Example 2:

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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:

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

Example 4:

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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:

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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

  1. First trim the input string. Check if the string’s length is 0 or string is NULL, return 0 as the result.

  2. Initialize index be 0, check the first character, if it’s + or - character, then update the flag be 1 or -1, and increase the index.

  3. Initialize res as double first, while the index is less than the length and the current character is a number, we update the result number be res = res * 10 + str.charAt(i) - '0'.

  4. Check if the res*flag number is equal or greater than the maximum integer or less or equal to the minimum integer, return the max integer or min integer accordingly. Otherwise, return res*flag as the result.

Solution

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class Solution {
    public int myAtoi(String str) {
        str = str.trim();
        if (str == null || str.length() == 0) return 0;
        int flag = 1;
        int i = 0;
        if (str.charAt(0) == '+') {
            i += 1;
        } else if (str.charAt(0) == '-') {
            flag = -1;
            i += 1;
        }
        
        double res = 0;
        while (i < str.length() && str.charAt(i) >= '0' && str.charAt(i) <= '9') {
            res = res * 10 + str.charAt(i) - '0';
            i += 1;
        }
        
        if (res*flag >= Integer.MAX_VALUE) {
            return Integer.MAX_VALUE;
        } else if (res*flag <= Integer.MIN_VALUE) {
            return Integer.MIN_VALUE;
        }
        return (int) res*flag;
    }
}