【剑指offer】把字符串转换成整数

private boolean isInvalid = false;

public int (String str) {
    if (str == null || str.length() < 1) {
        isInvalid = true;
        return 0;
    }

    char[] array = str.toCharArray();
    long result = 0;
    boolean minus = false;
    int index = 0;

    if (array[index] == '+')
        index++;
    else if (array[index] == '-') {
        minus = true;
        index++;
    }

    for (; index < array.length; index++) {
        if (array[index] < '0' || array[index] > '9') {
            isInvalid = true;
            return 0;
        }

        int flag = minus ? -1 : 1;
        result = result * 10 + flag * (array[index] - '0');

        if ((!minus && result > Integer.MAX_VALUE) || (minus && result < Integer.MIN_VALUE)) {
            isInvalid = true;
            return 0;
        }
    }

    return (int)result;
}