public String removeDuplicateLetters(String s) {

final Deque<Integer> dq = new ArrayDeque<>();

final int[] lastIdx = new int[26];

final int n = s.length();

final char[] w = s.toCharArray();

int mask = 0;

int last = 0;

for (int i = 0; i < n; i++) {

lastIdx[w[i] - 'a'] = i;

}

for (int i = 0; i < n; i++) {

while (!dq.isEmpty() && dq.getFirst() > w[i] - 'a' &&

!isSet(last, dq.getFirst()) && !isSet(mask, w[i] - 'a')) {

mask ^= 1 << dq.removeFirst();

}

if (!isSet(mask, w[i] - 'a')) {

mask ^= 1 << w[i] - 'a';

dq.addFirst(w[i] - 'a');

}

if (lastIdx[w[i] - 'a'] == i) {

last ^= 1 << w[i] - 'a';

}

}

final char[] res = new char[dq.size()];

for (int i = 0; !dq.isEmpty(); i++) {

res[i] = (char) (dq.removeLast() + 'a');

}

return new String(res);

}

private static boolean isSet(int mask, int idx) {

return (mask & (1 << idx)) != 0;

}