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 <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, s, t;
  fill(a, a+102, 0);

  for(int i = 1; i <= n; ++i){
    if(ch[i] == '#'){
      s = t = 0;
      p = i-1;
      while(0 < p && ch[p] == '#'){
	++s;
	--p;
      }
      if(p == 0) s = 100;

      p = i+1;
      while(p <= n && ch[p] == '#'){
	++t;
	++p;
      }
      if(p > n) t = 100;
      a[i] = min(s, t);
    } 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;
}

C++: #include<iostream>

char *chr, *peo;
int n,m;

void sitA(int i){
	for(int k=0; k<n; k++){
		if(chr[k] == '#'){
			chr[k] = 'A';
				break;
		}
	}
	return;
}

void sitB(int i){
	if(chr[n-1]=='#' && chr[n-2] != 'A'){
		chr[n-1] = 'B';
		return;
	}
	for(int k=n-2; k>0; k--){
		if(chr[k] == '#' && chr[k-1] != 'A' && chr[k+1] != 'A'){
			chr[k] = 'B';
			return;
		}
	}
	for(int k=0; k<n; k++){
		if(chr[k] == '#'){
			chr[k] = 'B';
			return;
		}
	}
}

void sitC(int i){
	if(i==0){
		if(n%2 == 0)
			chr[n/2] = 'C';
		else
			chr[(n-1)/2] = 'C';
		return;
	}
	for(int k=0; k<n; k++){
		if(chr[k] == '#')
			continue;
		if(k<n-1){
			if(chr[k+1] == '#'){
				chr[k+1] = 'C';
				return;
			}
		}
		if(k>0){
			if(chr[k-1] == '#'){
				chr[k-1] = 'C';
				return;
			}
		}
	}
}

void sitD(int i){
	if(i==0){
		chr[0] = 'D';
		return;
	}
	int ans,tmp=0,tmp1,tmp2;
	for(int k=0; k<n; k++){
		if(chr[k] == '#'){
			ans = k;
			break;
		}
	}
	for(int k=ans; k<n; k++){
		if(chr[k] != '#')
			continue;
		tmp1=0;
		tmp2=0;
		if(k<n-1){
			for(int t=k+1; t<n; t++){
				if(chr[t] != '#')
					break;
				tmp1++;
				if(t == n-1)
					tmp1 = n+1;
			}
		}
		else
			tmp1 = n+1;
		if(k>0){
			for(int t=k-1; t>=0; t--){
				if(chr[t] != '#')
					break;
				tmp2++;
				if(t == 0)
					tmp2 = n+1;
			}
		}
		else
			tmp2 = n+1;
		if(tmp1 > tmp2)
			tmp1 = tmp2;
		if(tmp1 > tmp){
			tmp = tmp1;
			ans = k;
		}
	}
	chr[ans] = 'D';
}

void solve(){
	char tmp;
	for(int i=0; i<m; i++){
		tmp = peo[i];
		switch(tmp){
		case 'a':
		case 'A':
			sitA(i);
			break;
		case 'b':
		case 'B':
			sitB(i);
			break;
		case 'c':
		case 'C':
			sitC(i);
			break;
		case 'd':
		case 'D':
			sitD(i);
			break;
		}
	}
	std::cout << chr << std::endl;
}

int main(){
	while(true){
		std::cin >> n >> m;
		if(!n && !m)
			return 0;
		chr = new char[n];
		peo = new char[m];
		for(int i=0; i<n; i++){
			chr[i] = '#';
			if(i>=m)
				chr[i] = '#';
			else
				std::cin >> peo[i];
		}
		solve();
		delete[] chr;
		delete[] peo;
	}
}

C++: #include<bits/stdc++.h>
#define INF (1e9)
#define N 102
using namespace std;
int n,m;
char a,ans[N];
 
void UpdateA(){
  for(int i=1;i<=n;i++)
    if(ans[i]=='#'){
      ans[i]='A';
      return;
    }
}
 
void UpdateB(){
  for(int i=n;i>0;i--){
    if(ans[i]!='#')continue;
    if(ans[i-1]!='A'&&ans[i+1]!='A'){
      ans[i]='B';
      return;
    }
  }
  for(int i=1;i<=n;i++)
    if(ans[i]=='#'){
      ans[i]='B';
      return;
    }
}
 
void UpdateC(){
  for(int i=1;i<=n;i++){
    if(ans[i]=='#')continue;
    if(ans[i+1]=='#'&&i+1<=n){
      ans[i+1]='C';
      return;
    }
    if(ans[i-1]=='#'&&1<=i-1){
      ans[i-1]='C';
      return;
    }
  }
  if(n%2)ans[(n+1)/2]='C';
  else ans[n/2+1]='C';
}
 
void UpdateD(){
  int d[2][N],cnt=INF,maxd;
  for(int i=0;i<2;i++)
    for(int j=1;j<=n;j++)d[i][j]=INF;
  for(int i=1;i<=n;i++){
    if(ans[i]!='#')cnt=0;
    else cnt++;
    d[0][i]=cnt;
  }
  cnt=INF;
  for(int i=n;i>=1;i--){
    if(ans[i]!='#')cnt=0;
    else cnt++;
    d[1][i]=min(cnt,d[0][i]);
  }
  maxd=0;
  for(int i=1;i<=n;i++)maxd=max(maxd,d[1][i]);
  if(maxd>=INF){
    ans[1]='D';
    return; 
  }else{
    for(int i=1;i<=n;i++){
      if(d[1][i]==maxd){
    ans[i]='D';
    return;
      }
    }
  }
}
 
int main(){
  while(1){
    cin>>n>>m;
    if(!n&&!m)break;
    for(int i=1;i<=n;i++)ans[i]='#';
    for(int i=0;i<m;i++){
      cin>>a;
      if(a=='A')UpdateA();
      if(a=='B')UpdateB();
      if(a=='C')UpdateC();
      if(a=='D')UpdateD();
    }
    for(int i=1;i<=n;i++)cout<<ans[i];
    cout<<endl;  
  }
  return 0;
}