String To Integer

String to integer, aka. atoi, is a classic problem. The corresponding problem is in fact a lot simple as you actually do not need to consider power start from E or e. So the only problem is to handle sign and number exceed. Integer that higher than Integer.MAX_VALUE and and lower than Integer.MIN_VALUE will be cut off and just return the corresponding limit.

Here are two different solutions:

public class StringToInteger {
    public int atoi(String str) {
        char[] strchar = str.toCharArray();
        int i = 0;
        while (i < strchar.length && strchar[i] == ' ') i++;
        long sign = 1;
        long value = 0;

        if (i >= strchar.length) return 0;

        if (strchar[i] == '-') {
            sign = -1;
            i++;
        }
        else if (strchar[i] == '+') i++;

        if (i < strchar.length && (strchar[i] < '0' || strchar[i] > '9')) return 0;

        while (i < strchar.length && strchar[i] >= '0' && strchar[i] <= '9') {
            value *= 10;
            value += (long)(strchar[i] - '0');
            i++;
            if (value > 2147483647) break;
        }

        value = Math.abs(value) * sign;

        if (value > 2147483647) return 2147483647;
        if (value < -2147483648) return -2147483648;
        return (int)value;
    }
}

public class StringToInteger {
    public int myAtoi(String string) {
        long sign = 1;
        long integer = 0;
        int state = 0;
        int i = 0;
        char[] str = string.toCharArray();

        while (i < str.length) {
            char ch = str[i];
            if (state == 0) {
                // this is sign mode
                if (ch == '-') {
                    sign = -1;
                    state = 1;
                    i++;
                }
                else if (ch == '+') {
                    state = 1; // switch to integer mode, so -+ and  +- is not valid
                    i++; // skip plus sign or spaces in the head
                }
                else if (ch == ' ') i++;
                else if (ch >= 0 && ch <= '9') state = 1; // switch to integer mode
                else if (ch == '.') {
                    state = 2; // swich to decimal mode
                    i++;
                } else break; // unexcepted char

            } else if (state == 1) {
                // this is integer mode
                if (ch >= '0' && ch <= '9') {
                    integer = integer * 10 + (ch - '0');
                    if (integer > -(long)Integer.MIN_VALUE) break;
                } else if (ch == '.') {
                    state = 2; // switch to decimal mode
                } else if (ch == 'E' || ch == 'e') {
                    state = 3; // switch to power mode
                } else break; // unexcepted char

                i++;
            }
        }

        long res = sign * integer;
        if (res > Integer.MAX_VALUE) return Integer.MAX_VALUE;
        else if ( res < Integer.MIN_VALUE) return Integer.MIN_VALUE;

        return (int)res;
    }
}