public String removeDuplicateLetters(String s) {

int[] result =new int[26];

for(int i=0;i<s.length();i++){

result[s.charAt(i)-97]=i;

}

boolean[] visited = new boolean[26];

Stack<Character> stack = new Stack <>();

for(int i=0;i<s.length();i++){

char c = s.charAt(i);

if(visited[c-'a']){

continue;

}

while(!stack.isEmpty () && stack.peek()>c && result[stack.peek () - 'a']>i){

visited[stack.peek () - 'a'] = false;

stack.pop ();

}

// System.out.println(stack+"\t"+c);

stack.push(c);

visited[c-'a']=true;

}

StringBuilder ans = new StringBuilder ();

while (!stack.isEmpty ()) {

ans.append (stack.pop());

}

return ans.reverse().toString ();

}

}