[LeetCode] 8. String to Integer (atoi)

Implement atoi to convert a string to an integer.

public class Solution {
    public int myAtoi(String s) {
        if (s == null || s.length() < 1) {
            return 0;
        }
        int i = 0;
        while (i < s.length() && s.charAt(i) == ' ') {
            i++;
        }
        if (i == s.length()) {
            return 0;
        }
        boolean isN = false;
        if (s.charAt(i) == '-') {
            isN = true;
            i++;
        } else if (s.charAt(i) == '+') {
            i++;
        }
        double r = 0;
        for (; i < s.length(); i++) {
            if (s.charAt(i) < '0' || s.charAt(i) > '9') {
                break;
            }
            r = r * 10 + (s.charAt(i) - '0');
        }
        if (isN) {
            r = -r;
            if (r < Integer.MIN_VALUE) {
                return Integer.MIN_VALUE;
            }
        } else if (r > Integer.MAX_VALUE) {
            return Integer.MAX_VALUE;
        }
        return (int)r;
    }
}