a_139_1stNon_repeating.java

import java.util.* ;

public class a_139_1stNon_repeating {

    public static void printNonRepeating (String str){
        int freq[] = new int[26] ;    // 'a' -> 'z'
        Queue<Character> q = new LinkedList<>() ;

        for(int i=0 ; i<str.length() ; i++ ){
            char ch = str.charAt(i) ;
            q.add(ch) ;
            freq[ch-'a']++ ;

            while(!q.isEmpty() && freq[q.peek()-'a'] > 1 ) {
                q.remove() ;
            }

            if(q.isEmpty()) {
                System.out.print(-1 + " ");
            }
            else{
                System.out.print(q.peek() + " ");
            }

        }

        System.out.println();

    }

    public static void main(String[] args) {
        String str = "aabccxb" ;
        printNonRepeating(str);
        

    }

}