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++: /*
 * src.cpp
 *
 *  Created on: 2016/07/27
 *      Author: joi
 */

#include <bits/stdc++.h>

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,a) FOR(i,0,a)

using namespace std;

const int MAX_N=1e2,INF=1e8;

int n,m;

char chairs[MAX_N+1];

int dist(int a){
	int res=INF;
	REP(i,n){
		if (chairs[i]!='#'){
			res=min(res,abs(i-a));
		}
	}
	return res;
}

int main(){
	do{
		cin>>n>>m;
		if (n || m){
			cin.ignore();
			REP(i,n){
				chairs[i]='#';
			}
			chairs[n]=0;
			REP(i,m){
				char c;
				cin>>c;
				switch (c){
				case 'A':{
					REP(i,n){
						if (chairs[i]=='#'){
							chairs[i]='A';
							break;
						}
					}
					break;
				}
				case 'B':{
					int v=-1;
					for (int i=n-1;i>=0;i--){
						if (chairs[i]=='#'){
							if (i!=0){
								if (chairs[i-1]=='A'){
									continue;
								}
							}
							if (i+1!=n){
								if (chairs[i+1]=='A'){
									continue;
								}
							}
							v=i;
							break;
						}
					}
					if (v!=-1){
						chairs[v]='B';
					}else{
						REP(i,n){
							if (chairs[i]=='#'){
								chairs[i]='B';
								break;
							}
						}
					}
					break;
				}
				case 'C':{
					if (i==0){
						if (n%2==1){
							chairs[(n+1)/2-1]='C';
						}else{
							chairs[n/2]='C';
						}
					}else{
						FOR(i,1,n){
							if (chairs[i]=='#' && chairs[i-1]!='#'){
								chairs[i]='C';
								break;
							}
						}
					}
					break;
				}
				case 'D':{
					int a=0,d=dist(0);
					FOR(i,1,n){
						if (dist(i)>d){
							a=i;
							d=dist(i);
						}
					}
					chairs[a]='D';
					break;
				}
				}
			}
			cout<<chairs<<endl;
		}
	}while(n || m);
	return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cstring>
#define INF 100000
using namespace std;
int n;
char seki[101],c;
void SetA(short);
void SetB();
void SetC(bool);
void SetD(bool);
int main(){
  int m;
  while(cin >> n >> m && n||m){
    memset(seki,'#',sizeof(seki));
    for(int i=0;i<m;i++){
      cin >> c;
      if(c=='A') SetA(1);
      else if(c=='B') SetB();
      else if(c=='C') SetC((i==0 ? false : true));
      else SetD((i==0 ? false : true));
    }
    for(int i=0;i<n;i++) cout << seki[i];
    cout << endl;
  }
}
void SetA(short ab){  // とりあえず左の席
  for(int i=0;i<n;i++){
    if(seki[i]=='#'){
      seki[i] = (ab==1 ? 'A' : (ab==2 ? 'B' : 'D'));
      return;
    }
  }
}
void SetB(){  // 隣にAがいなければHappy
  for(int i=n-1;i>=0;i--){
    if(seki[i-1]!='A' && seki[i]=='#' && seki[i+1]!='A'){
      seki[i] = 'B';
      return;
    }
  }
  SetA(2);
}
void SetC(bool k){ // 人が好き
  if(!k){
    seki[(n%2==1 ? (n+1)/2 : (n/2+1)) -1] = 'C';
    return;
  }
  for(int i=0;i<n;i++){
    if(seki[i] != '#'){
      if(seki[i+1] == '#'){
	seki[i+1] = 'C';
	return;
      }
      if(seki[i-1] == '#'){
	seki[i-1] = 'C';
	return;
      }
    }
  }
}
void SetD(bool k){ // 人が嫌い
  if(!k){
    seki[0] = 'D';
    return;
  }
  int MAX=0,j,m,MAXi;
  for(int i=0;i<n;i++){
    if(seki[i]=='#'){
      for(j=i-1;j>=0;j--) if(seki[j]!='#') break;
      for(m=i+1;m<n;m++) if(seki[m]!='#') break;
      if(j==-1) j = INF;
      if(m==n) m = INF;
      if(j==INF){
	if(k-i > MAX) MAX = m-i,MAXi=i,MAXi=i;
      } else if(m==INF){
	if(i-j > MAX) MAX = i-j,MAXi=i;
      } else{
	if(min(m-i,i-j) > MAX) MAX=min(m-i,i-j),MAXi=i;
      }
    }
  }
  if(MAX==0) SetA(3);
  else seki[MAXi] = 'D';
}