STack.txt

//First non Repeating



public class Solution {
    public String solve(String A) {
        StringBuilder ans=new StringBuilder();
     Queue<Character> q=new LinkedList<>();
    
    int arr[]=new int[26];
    for(int i=0;i<A.length();i++) {
        char ch=A.charAt(i);
        arr[ch-'a']++;
        q.add(ch);
        if(i==0 || arr[q.peek()-'a']==1) {
             ans.append(q.peek());
        }else {
            while(!q.isEmpty()) {
                if(arr[q.peek()-'a']!=1) {
                    q.poll();
                }else {
                    break;
                }
            }
             if(q.isEmpty()) ans.append('#');
             else ans.append(q.peek());
        }
    }
      return ans.toString();
    }
}