string-to-integer-atoi.js

// 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.
//


/**
 * @param {string} str
 * @return {number}
 */
var myAtoi = function(str) {
    var r = str.match(/^\s*([+-]?\d+)/),
        n = (r && +r[1]);
    return  !n ? 0 :
            n > 2147483647 ? 2147483647 :
            n < -2147483648 ? -2147483648 :
            n;
};