Go: package main

import (
	"bufio"
	"os"
	"strings"
	"strconv"
	"fmt"
	)

var scanner = bufio.NewScanner(os.Stdin)
const INF = 0x7FFFFFFF

func main() {
	for scanner.Scan() {
		buf := strings.Split(scanner.Text(), " ")
		n, _ := strconv.Atoi(buf[0])
		m, _ := strconv.Atoi(buf[1])
		if n == 0 && m == 0 {
			break
		}
		chairs := make([]string, n)
		for i := range chairs {
			chairs[i] = "#"
		}
		sit := func(who string) {
			if who == "A" {
				chairs[getStringIndex(chairs, "#")] = "A" // A国の人は左から見て行って空いている席を見つけ次第座る
				// log.Println(chairs)
			} else if who == "B" {
				f := true
				for i := n-1; i > -1; i-- {
					// B国の人はA国人の隣以外で、右端から空いている椅子に座る
					if chairs[i] == "#" && (i == 0 || chairs[i-1] != "A") && (i == n-1 || chairs[i+1] != "A") {
								chairs[i] = "B"
								f = false
								break
					}
				}
				// A国人の隣しか空いていない場合、我慢して左端から空いている椅子に座る
				if f {
					chairs[getStringIndex(chairs, "#")] = "B"
				}
				// log.Println(chairs)
			} else if who == "C" { // C国人は人の隣に座りたい
				for i := 0; i < n; i++ {
					if chairs[i] != "#" {
						if i + 1 <= n-1 && chairs[i+1] == "#" {
							chairs[i+1] = "C"
							break
						} else if i - 1 >= 0 && chairs[i-1] == "#" {
							chairs[i-1] = "C"
							break
						}
					}
				}
				// log.Println(chairs)
			} else { // who == "D"
				// D国の人は人から離れて座りたい
				// どうしても誰かの隣に座らなければならない場合、その中で一番左側にある椅子に座る
				scores := make([]int, n)
				for i := range scores {
					scores[i] = INF
				}
				for i, v := range chairs {
					if v == "#" {
						scores[i]++
					}
				}
				for i := n-1; i > -1; i-- {
					tmp := 0
					if chairs[i] == "#" {
						tmp++
					} else {
						tmp = 0
					}
					scores[i] = min(scores[i], tmp)
				}
				chairs[getIntIndex(scores, getMax(scores))] = "D"
				// log.Println(chairs)
			}
		}
		scanner.Scan()
		if first := scanner.Text(); first == "A" || first == "D" {
			chairs[0] = first
		} else if first == "B" {
			chairs[len(chairs)-1] = "B"
		} else {
			chairs[n/2] = "C"
		}
		// log.Println(chairs)
		for i := 0; i < m-1; i++ {
			scanner.Scan()
			sit(scanner.Text())
		}
		fmt.Println(strings.Join(chairs, ""))
	}
}

func getStringIndex(chairs []string, who string) int {
	for i, v := range chairs {
		if v == who {
			return i
		}
	}
	return -1
}

func getIntIndex(scores []int, target int) int {
	for i, v := range scores {
		if v == target {
			return i
		}
	}
	return -1
}

func min(a, b int) int {
	if a > b {
		return b
	}
	return a
}

func getMax(scores []int) int {
	tmp := -1
	for _, v := range scores {
		if tmp < v {
			tmp = v
		}
	}
	return tmp
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<algorithm>
#include<map>
#include<set>
#include<utility>
#include<cmath>
#include<cstring>
#include<queue>
#include<cstdio>
#include<sstream>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb push_back
#define mp make_pair
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
const double EPS=1e-10;
const int inf=1e8;
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pii;
int main(){
	int n,m;
	while(cin>>n>>m,n){
		vector<string>in(n,"");
		rep(q,m){
			string s;cin>>s;
			if(s=="A"){
				rep(i,n)if(in[i]==""){
					in[i]="A";
					break;
				}
			}else if(s=="B"){
				vector<string>tmp=in;
				rep(i,n)if(in[i]=="A"){
					if(i)tmp[i-1]="!";
					if(i!=n-1)tmp[i+1]="!";
				}
				int I=inf;
				for(int i=n-1;i>=0;i--)if(tmp[i]==""){I=i;break;}
				if(I==inf){
					rep(i,n)if(in[i]==""){
						I=i;
						break;
					}
				}
				in[I]="B";
			}else if(s=="C"){
				int I=inf;
				rep(i,n)if(in[i]==""){
					if((i&&in[i-1]!="")||(i!=n-1&&in[i+1]!="")){
						I=i;
						break;
					}
				}
				if(I==inf)I=n/2;
				in[I]="C";
			}else{
				if(!q){
					in[0]="D";
					continue;
				}
				vi t;
				rep(i,n)if(in[i]!="")t.pb(i);
				int ma=-1,mai=0;
				if(t[0]!=0){ma=t[0];mai=0;}
				rep(i,t.size()-1)if(t[i+1]-t[i]>1){
					if(ma<(t[i+1]-t[i])/2){
						ma=(t[i+1]-t[i])/2;
						mai=t[i]+(t[i+1]-t[i])/2;
					}
				}
				if(t[t.size()-1]!=n-1&&ma<n-t[t.size()-1]-1){
					mai=n-1;
				}
				in[mai]="D";
			}
		}
		rep(i,n)cout<<(in[i]==""?"#":in[i]);
		cout<<endl;
	}
}

C++: #include<iostream>
#include<algorithm>
using namespace std;

int main(){
  int n,m;

  for(;;){
    cin >> n >> m;
    if(!n && !m)break;

    string c;
    for(int i=0;i<n;i++)c += '#';

    for(int i=0;i<m;i++){
      char g;
      cin >> g;

      int pos;
      if(g == 'A'){
	pos = 0;
	while(c[pos]!='#')pos++;
      }else if(g == 'B'){
	pos = n-1;
	while(pos>=0){
	  if(c[pos]=='#'){
	    if( (pos==n-1 || c[pos+1]!='A') && (pos==0 || c[pos-1]!='A') )break;
	  }
	  pos--;
	}
	if(pos<0){
	  pos = 0;
	  while(c[pos]!='#')pos++;
	}
      }else if(g == 'C'){
	if(!i)pos = n/2;
	else{
	  pos = 0;
	  while(pos<n){
	    if(c[pos]!='#'){
	      if(pos<n-1 && c[pos+1]=='#'){
		pos++;
		break;
	      }
	      if(pos>0 && c[pos-1]=='#'){
		pos--;
	      break;
	      }
	    }
	    pos++;
	  }
	}
      }else{
	pos = 0;
	if(i){
	  int dis = 0;
	  int tmp = -1;
	  while(pos<n){
	    if(c[pos]=='#'){
	      int l = 1,r = 1;
	      while(pos-l>=0 && c[pos-l]!='#')l++;
	      while(pos+r<n && c[pos+r]!='#')r++;
	      if(pos-l<0){
		if(dis<r){
		  dis = r;
		  tmp = pos;
		}
	      }else if(pos+r>=n){
		if(dis<l){
		  dis = l;
		  tmp = pos;
		}
	      }else{
		if(dis<min(l,r)){
		  dis = min(l,r);
		  tmp = pos;
		}
	      }
	    }
	    pos++;
	  }
	  pos = tmp;
	}
      }
      c[pos] = g;
    }

    cout << c << endl;
  }
}

C++: #include <iostream>
#include <algorithm>

using namespace std;

int n, m;
char ch[102];

void funcA()
{
  for(int i = 1; i <= n; ++i)
    if(ch[i] == '#'){
      ch[i] = 'A';
      break;
    }

  return ;
}

void funcB()
{
  for(int i = n; i > 0; --i)
    if(ch[i] == '#' && ch[i-1] != 'A' && ch[i+1] != 'A'){
      ch[i] = 'B';
      return ;
    }

  for(int i = 1; i <= n; ++i)
    if(ch[i] == '#'){
      ch[i] = 'B';
      break;
    }

  return ;
}

void funcC()
{
  bool f = true;
  for(int i = 1; i <= n && f; ++i)
    if(ch[i] != '#') f = false;

  if(f){
    int t = (n%2 == 0)? (n/2+1): ((n+1)/2);
    ch[t] = 'C';
    return ;
  }

  for(int i = 1; i <= n; ++i)
    if(ch[i] != '#'){
      if(i+1 <= n && ch[i+1] == '#'){
	ch[i+1] = 'C';
	break;
      }
      if(i-1 > 0 && ch[i-1] == '#'){
	ch[i-1] = 'C';
	break;
      }
    }

  return ;
}

void funcD()
{
  int a[102], p;
  fill(a, a+102, 0);

  for(int i = 1; i <= n; ++i){
    if(ch[i] == '#'){
      p = i-1;
      while(0 < p && ch[p] == '#'){
	a[i]++;
	--p;
      }
      if(p == 0) a[i] = 0;

      p = i+1;
      while(p <= n && ch[p] == '#'){
	a[i]++;
	++p;
      }
      if(p > n) a[i] -= (p-i-1);
    } else {
      a[i] = -1;
    }
  }

  p = 1;
  for(int i = 2; i <= n; ++i)
    if(a[i] > a[p]) p = i;
  ch[p] = 'D';

  return ;
}

int main()
{
  char p;

  while(cin>>n>>m && (n+m)){
    fill(ch, ch+102, '#');
    while(m--){
      cin >> p;
      if(p == 'A') funcA();
      if(p == 'B') funcB();
      if(p == 'C') funcC();
      if(p == 'D') funcD();
    }

    for(int i = 1; i <= n; ++i) cout << ch[i];
    cout << endl;
  }

  return 0;
}