Day41_LeetCode316.java

class Solution {
    public String removeDuplicateLetters(String s) {
        int[] lastIndex = new int[26];

        for(int i=0;i<s.length();i++){
            char ch = s.charAt(i);
            int idx = (int)(ch-'a');

            lastIndex[idx]= i ;
        }

        boolean[] present = new boolean[26];

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

        for(int i=0;i<s.length();i++){
              char ch = s.charAt(i);
            int idx = (int)(ch-'a');

            if(present[idx]==false){
                while(st.size()>0 && st.peek()>ch && lastIndex[(int)(st.peek()-'a')]>i){
                    present[(int)(st.peek()-'a')]=false;
                    st.pop();
                }

                st.push(ch);
                present[idx]=true ;
            }
        }

        StringBuilder sb = new StringBuilder("");

        while(st.size()>0){
            sb.append(st.pop());
        }

        return sb.reverse().toString();
    }
}