String to Integer II（lintcode 54)

Description

Implement function atoi to convert a string to an integer.

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.

Example

"10" => 10

"-1" => -1

"123123123123123" => 2147483647

"1.0" => 1

Interface

public class Solution {
    /**
     * @param str: A string
     * @return An integer
     */
    public int atoi(String str) {
        // write your code here
    }
}

Idea

The difficulty is a lot of conditions have to be considered.

Solution

public class Solution {
    /**
     * @param str: A string
     * @return An integer
     */
    public int atoi(String str) {
        // write your code here
        if (str == null || str.length() < 1) {
            return 0;
        }

        str = str.trim();

        char flag = '+';
        int i = 0;
        if (str.charAt(i) == '-') {
            flag = '-';
            ++i;
        } else if (str.charAt(i) == '+') {
            ++i;
        }

        double res = 0;
        while (i < str.length() && str.charAt(i) >= '0' && str.charAt(i) <= '9') {
            res = res * 10 + str.charAt(i) - '0';
            ++i;
        }

        if (flag == '-') {
            res = -res;
        }
        if (res > Integer.MAX_VALUE) {
            return Integer.MAX_VALUE;
        }
        if (res < Integer.MIN_VALUE) {
            return Integer.MIN_VALUE;
        }
        return (int)res;
    }
}