StringtoInteger.java

package learn.freq05;

//Implement atoi to convert a string to an integer.
//
//Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.
//
//Notes: It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.
//
//spoilers alert... click to show requirements for atoi.
//Requirements for atoi:
//
//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. If the correct value is out of the range of representable values, INT_MAX (2147483647) or INT_MIN (-2147483648) is returned.

//这题考研在要求很多的情况下 是否能做到条理清晰 有条不紊
public class StringtoInteger {
    public static int atoi(String str) {
        str = str.trim();
        long value = 0;
        if (str.equals("")) {
            return 0;
        }
        boolean isPositive = true;
        for (int i = 0; i < str.length(); i++) {
            //在第一位的状态下先处理第一位  注意这种写法 很巧妙 逻辑很清晰
            if (i == 0 && (str.charAt(i) == '+' || str.charAt(i) == '-')) {
                if (str.charAt(i) == '-') {
                    isPositive = false;
                }
                continue;
            } //continue的意思是 continue下面的都不执行了 直接进行下一轮循环
            //你看 利用ASCII 码比大小,你不用吧具体的码背出来
            //只要比0小 比9大的 都不是数字,避免了使用正则表达式
            //而且只有string可以用正则，char不可以直接用
            if (str.charAt(i) < '0' || str.charAt(i) > '9') {
                break;
            }
            //这里更巧妙了 因为是从左到右读取 上次的值x10+本次的值 就是current值
            //eg：123=  /1/1*10+2/12x10+3 =123
            value = 10 * value + str.charAt(i) - '0';
        }
        if (isPositive == false) {
            value = value * -1;
        }
        if (value > Integer.MAX_VALUE) {
            return Integer.MAX_VALUE;
        } else if (value < Integer.MIN_VALUE) {
            return Integer.MIN_VALUE;
        } else {
            return (int) value;
        }//反正已经在正常int范围内了

    }


    public static void main(String[] args) {
        System.out.print(atoi("  -233"));
    }

}