import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Iterator;
import java.util.Stack;


public class problem1654 {

    public static void main(String[] args) throws IOException {
    BufferedReader inp = new BufferedReader(new InputStreamReader(System.in));
    String str=inp.readLine();
    Stack<Character> st=new Stack<Character>();
    st.push(str.charAt(0));
    String s="";
    for(int i=1;i<str.length();i++){
    if(!st.empty()&&st.peek()==str.charAt(i))
        st.pop();
    else{
    st.push(str.charAt(i));

    }
    }



while(!st.empty()){
    s=s+st.pop();
}
int n=s.length();
for(int i=0;i<n;i++){
    System.out.print(s.charAt(n-i-1));


    }

    }}

When i used stack(from STL) in C++, i'd got TL8.
But after i realzed stack by using array, i got AC

Don't use :
while(!st.empty()){
    s=s+st.pop();
}
--------
Use this phrase at the end of your Code:
Iterator<Character> b = st.iterator();
        while(b.hasNext()){
            System.out.print(b.next());
        }

your reply help me very much!3Q