StringtoIntegerAtoi.java

package medium;
/**
 * 
 * ClassName: StringtoIntegerAtoi
 * @author chenyiAlone  
 * Create Time: 2018/12/15 15:01:32
 * Description: No.8
 * 思路:
 *      1. tirm() 去掉首尾空格
 *      2. 取出符号
 *      3. 逐位累加，判断边界
 *
 * 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.
 *
 */
public class StringtoIntegerAtoi {
    // +-  越界 
    public int myAtoi(String str) {
        
        int sign = 1;
        int start = 0;
        long ans = 0;
        
        str = str.trim();
        if (str.length() == 0) return 0; // 必须放在取出首位空字符之后
        char firstChar = str.charAt(0);
        if (firstChar == '-') {
            sign = -1;
            start++;
        } else if (firstChar == '+'){
            start++;
        }
        for (int i = start; i < str.length(); i++) {
            if (!Character.isDigit(str.charAt(i))) {
                return (int)ans * sign;
            }
            ans = ans * 10 + (str.charAt(i) - '0');
            if (sign == 1 && ans > Integer.MAX_VALUE) return Integer.MAX_VALUE;
            if (sign == -1 && ans * -1 < Integer.MIN_VALUE) return Integer.MIN_VALUE;
        }
        
        return (int)ans * sign;
    }
    public static void main(String[] args) {
//        String str = "";
//        System.out.println(str.charAt(1));
//        System.out.println(str.length());
//        System.out.println(new StringtoIntegerAtoi().myAtoi(str));
        long i = 1L << 33;
        System.out.println(i);
        System.out.println((int)i);
        i = -(1L << 33);
        System.out.println(i);
        System.out.println(Integer.MAX_VALUE);
    }
}