1.3

permutationString.java

/**
 * Created by Readman on 6/16/14.
 */
public class permutationString {
    /*
    time: n
    space : 1
    * */
    public static boolean perm (String s1, String s2) {
        if (s1 == null || s2 == null)
            return false;
        if (s1.isEmpty() && s2.isEmpty())
            return true; // two empty strings are permutation.
        if (s1.isEmpty() || s2.isEmpty())
            return false;
        if (s1.length() != s2.length())
            return false;
        int[] test = new int[256];
        for (char c : s1.toCharArray()) {
            test[c] += 1;
        }
        for (char c : s2.toCharArray()) {
            test[c] -= 1;
        }
        for (int i : test) {
            if (i != 0)
                return false;
        }
        return true;
    }

    public static void main(String[] args) {
        System.out.println(perm("acc", "ccc"));
    }
}

line 22 to 28 can be changed into
for(char c : str2.toCharArray()) {
if (--letters[c] < 0) {
return false;
}
}

Is this better? :)

I think in 14, if should be change to if(s1.isEmpty()+ s2.isEmpty()==0);