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>
using namespace std;
#define BLANK '#'
int main()
{
  int n,m;
  int i,j,blank,cnt,max;
  bool f;
  char c;
  vector<char> seat;
  while(cin>>n>>m,n|m){
    seat.resize(n);
    for(i=0;i<n;i++)seat[i]=BLANK;
    while(m-->0){
      cin>>c;
      switch(c){
	// A
	case 'A':
	  for(i=0;i<n;i++)
	    if(seat[i]==BLANK){seat[i]=c;break;}
	  break;
	// B
	case 'B':
	  for(i=n;i-->0;){
	    if(seat[i]==BLANK){
	      f=true;
	      if(i+1<n&&seat[i+1]=='A')f=false;
	      if(i>0&&seat[i-1]=='A')f=false;
	      blank=i;
	      if(f){seat[blank]=c;break;}
	    }
	  }
	  if(seat[blank]==BLANK)seat[blank]=c;
	  break;
	// C
	case 'C':
	  f=true;
	  for(i=0;i<n;i++)if(seat[i]!=BLANK)f=false;
	  if(f){seat[n/2]=c;break;}
	  for(i=0;i<n;i++){
	    if(seat[i]!=BLANK){
	      if(i+1<n&&seat[i+1]==BLANK){seat[i+1]=c;break;}
	      if(i>0&&seat[i-1]==BLANK){seat[i-1]=c;break;}
	    }
	  }
	  break;
	// D
	case 'D':
	  max=-1;
	  blank=0;
	  for(i=0;i<n;i++){
	    if(seat[i]==BLANK){
	      f=false;
	      for(cnt=0,j=i;j-->0;cnt++)if(seat[j]!=BLANK){f=true;break;}
	      if(f&&max<cnt){max=cnt;blank=i;}
	      f=false;
	      for(cnt=0,j=i;++j<n;cnt++)if(seat[j]!=BLANK){f=true;break;}
	      if(f&&max<cnt){max=cnt;blank=i;}
	    }
	  }
	  seat[blank]=c;
	  break;
      }
    }
    for(i=0;i<n;i++)cout<<seat[i];
    cout<<endl;
  }
}

C++: #include<iostream>

using namespace std;

int n,m;
char seat[100];

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

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

	for(int i=0;i<n;i++){
		if(seat[i]=='#'){ seat[i]='B'; return; }
	}
}

void sitC()
{
	for(int i=0;i<n;i++){
		if(seat[i]!='#'){
			if(i+1<n &&seat[i+1]=='#'){ seat[i+1]='C'; return; }
			if(0<=i-1&&seat[i-1]=='#'){ seat[i-1]='C'; return; }
		}
	}
	seat[n/2]='C';
}

void sitD()
{
	int dmax=0,imax;
	for(int i=0;i<n;i++){
		if(seat[i]=='#'){
			int disL=0,disR=0;
			for(int j=i;seat[j]=='#';j--,disL++){
				if(j==0){ disL=10000; break; }
			}
			for(int j=i;seat[j]=='#';j++,disR++){
				if(j==n-1){ disR=10000; break; }
			}
			int dis=min(disL,disR);
			if(dmax<dis)	dmax=dis,imax=i;
		}
	}
	seat[imax]='D';
}

int main()
{
	for(;cin>>n>>m,n||m;){
		for(int i=0;i<n;i++)seat[i]='#';
		for(int i=0;i<m;i++){
			char c;	cin>>c;
			switch(c){
				case 'A':	sitA();	break;
				case 'B':	sitB();	break;
				case 'C':	sitC();	break;
				case 'D':	sitD();	break;
			}
		}
		for(int i=0;i<n;i++)	cout<<seat[i];
		cout<<endl;
	}

	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int n,m;
char t[120];

void A(char a){
  int i=1;
  while(t[i]!=0) i++;
  t[i]=a;
}

void B(){
  for(int i=n;i>=1;i--){
    if(t[i]==0){
      t[i]='B';
      break;
    }
  }
}

void C(int cnt){
  if(cnt==0) {
    if(n%2==0) t[n/2+1]='C';
    else t[(n+1)/2]='C';
    return;
  }

  for(int i=1;i<=n;i++){
    if(t[i]!=0&&t[i+1]==0) {
      t[i+1]='C';
      break;
    }
    if(t[i]!=0&&t[i-1]==0){
      t[i-1]='C';
      break;
    }
  }
}

void D(int cnt){
  if(cnt==0){
    t[1]='D';
    return;
  }
  int mx=0,idx=0,c=0;
  for(int i=1;i<=n;i++) {
    if(t[i]==0) c++;
    else c=0;
    if(mx<c)mx=c,idx=i;
  }
  if(mx<=2){
    A('D');
    return ;
  }

  if(idx==n) t[n]='D';
  else   t[(idx-(idx-mx+1))/2+(idx-mx+1)]='D';
}

int main(){
  while(1){
    cin>>n>>m;
    if(!n&&!m)break;
    memset(t,0,sizeof(t));
    t[0]=t[n+1]=1;
    for(int i=0;i<m;i++){
      char a;
      cin >> a;
      if(a=='A')A(a);
      if(a=='B')B();
      if(a=='C')C(i);
      if(a=='D')D(i);
    }

    for(int i=1;i<=n;i++)if(t[i]==0)t[i]='#';
    for(int i=1;i<n;i++) cout <<t[i];
    cout <<t[n]<<endl;
  }
  return 0;
}