Day26

class Solution {
    public String removeDuplicateLetters(String s) {
        int n=s.length();
        int lastOcurrence[]=new int[26];
        for(int i=0;i<n;i++)
           lastOcurrence[s.charAt(i)-'a']=i;

        boolean visited[]=new boolean[26];
        Stack<Integer>stack=new Stack<>();

        for(int i=0;i<n;i++){
            int currChar=s.charAt(i)-'a';
            if(visited[currChar])
              continue;
            while(!stack.isEmpty() && stack.peek()>currChar 
                  && i<lastOcurrence[stack.peek()])
            {
                visited[stack.pop()]=false;
            }
            stack.push(currChar);
            visited[currChar]=true;
        }
        StringBuilder str=new StringBuilder();
        while(!stack.isEmpty()){
            str.append((char)( stack.pop()+'a' ));
        }
        return str.reverse().toString();
        
    }
}