Solution.java

class Solution {
    public boolean isUnique(String astr) {
        int mask = 0;
        for (char c : astr.toCharArray()) {
            int i = c - 'a';
            if (((mask >> i) & 1) == 1) {
                return false;
            }
            mask |= 1 << i;
        }
        return true;
    }
}