8. String to Integer

• 检查空字符串
• 设置遍历指针index
• 去掉前面的空格，注意中间不用去掉，如+0 123，不是整数
• 边界检查后，检查空格之后的第一个符号位（如果存在）
• 进入循环，判断是否为数字，判断是否溢出
• 返回sign * res

public static int myAtoi3(String str) {
    // check null and empty
    if (str == null || str.isEmpty()) return 0;

    int index = 0, sign = 1, res = 0;
    // this while loop can be replaced as str = str.trim();
    while (index < str.length() && str.charAt(index) == ' ') {
        index++;
    }

    // check boundary
    if (index == str.length()) return 0;

    if (str.charAt(index) == '+' || str.charAt(index) == '-') {
        sign = (str.charAt(index) == '-') ? -1 : 1;
        index++;
    }

    while (index < str.length()) {
        int digit = str.charAt(index) - '0';
        if (digit < 0 || digit > 9) break;

        // check overflow
        if (Integer.MAX_VALUE / 10 == res && Integer.MAX_VALUE % 10 < digit || Integer.MAX_VALUE / 10 < res)
            return (sign == -1) ? Integer.MIN_VALUE : Integer.MAX_VALUE;

        res = res * 10 + digit;
        index++;
    }
    return sign * res;
}

换成char array

public static int myAtoi(String str) {
    int index = 0;
    char[] c = str.toCharArray();
    if (c.length == 0) return 0;

    while (index < c.length && c[index] == ' ')
        index++;
    if (index == c.length) return 0;

    // check sign
    int sign = 1;
    if (c[index] == '-' || c[index] == '+') {
        sign = c[index] == '-' ? -1 : 1;
        index++;
    }

    int res = 0;
    for (; index < c.length; index++) {
        int digit = c[index] - '0';
        if (digit < 0 || digit > 9) break; // not a digit

        // check overflow: && > ||
        if ((Integer.MAX_VALUE / 10 == res && Integer.MAX_VALUE % 10 < digit) || Integer.MAX_VALUE / 10 < res)
            return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;

        res = res * 10 + digit;
    }
    return res * sign;
}