String to Integer (atoi)

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.

public int myAtoi(String str) {
    if(str == null) return 0;
    //清除字符串首尾空格
    str = str.trim();
    if(str.length() == 0) return 0;
    
    boolean positive = true;
    int index = 0;
    long result = 0;
    
    //确定符号位并向后扫描一位
    if(str.charAt(index) == '-') {
        ++index;
        positive = false;
    } else if(str.charAt(index) == '+')
        ++index;
       
    //从符号位向后扫描，注意result = result * 10 + (c - '0')
    //这种字符串转数字的方法，这种方法只需要从前向后扫描一次
    //每次只需要一次乘法和一次加法（不用分别计算结果和基）
    while(index < str.length()) {
        char c = str.charAt(index);
        if(c >= '0' && c <= '9') {
            result = result * 10 + (c - '0');
            ++index;
        } else
        //如果遇到非数字字符，提前退出循环
            break;
        //超出Integer表示范围时，返回边界值
        if(positive && result >= Integer.MAX_VALUE) return Integer.MAX_VALUE;
        if(!positive && (-result) < Integer.MIN_VALUE) return Integer.MIN_VALUE;
    }
    return positive ? (int) result : (int)(-result);
}