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 <string>
#include <vector>
using namespace std;

const int N_MAX = 100;

void sitA( char* seat, int n )
{
	int i = 0;
	while( seat[ i ] != '#' ){
		++i;
	}
	seat[ i ] = 'A';
}

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

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

void sitD( char* seat, int n )
{
	int est[ N_MAX ];
	int s, e;
	vector< int > seated;
	for( int i = 0; i < n; ++i ){
		if( seat[ i ] != '#' ){
			seated.push_back( i );
		}
	}
	if( seated.size() == 0 ){
		seat[ 0 ] = 'D';
		return;
	}
	e = seated[ 0 ];
	for( int i = e; i >= 0; --i ){
		est[ i ] = e - i;
	}
	for( int i = 1; i < seated.size(); ++i ){
		s = e;
		e = seated[ i ];
		for( int j = 0; j <= ( e - s - 1 ) / 2; ++j ){
			est[ s + j ] = est[ e - j ] = j;
		}
	}
	for( int i = e; i < n; ++i  ){
		est[ i ] = i - e;
	}
	int estMax = 0, estIdx = 0;
	for( int i = 0; i < n; ++i ){
		if( est[ i ] > estMax ){
			estMax = est[ i ];
			estIdx = i;
		}
	}
	seat[ estIdx ] = 'D'; 
}

void sit( char a, char* seat, int n )
{
	switch( a ){
		case 'A': sitA( seat, n ); break;
		case 'B': sitB( seat, n ); break;
		case 'C': sitC( seat, n ); break;
		case 'D': sitD( seat, n ); break;
	}
}

int main()
{
	int n, m;
	char seat[ N_MAX + 1 ];
	while( cin >> n >> m && ( n || m ) ){
		for( int i = 0; i < n; ++i ){
			seat[ i ] = '#';
		}
		seat[ n ] = '\0';

		for( int i = 0; i < m; ++i ){
			char a;
			cin >> a;
			sit( a, seat, n );
		}
		cout << seat << endl;
	}
	return 0;
}

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

#define REP(i,n,m) for(int i=n;i<m;i++)
#define rep(i,n) REP(i,0,n)

int n;
char s[104];

int A(void){
	rep(i,n) if(s[i] == '#') return i;
	return -1;
}

int B(void){
	for(int i=n-1;i>=0;i--){
		if(s[i] == '#'){
			int l = i - 1;
			int r = i + 1;
			if(l >= 0 && s[l] == 'A') continue;
			if(r < n && s[r] == 'A') continue;

			return i;
		}
	}

	return -1;
}

int C(void){
	rep(i,n){
		if(s[i] != '#'){
			int l = i - 1;
			int r = i + 1;
			if(r < n && s[r] == '#') return r;
			if(l >= 0 && s[l] == '#') return l;
		}
	}

	return -1;
}

int D(void){
	int res = -1;
	int maxDist = 0;

	rep(i,n){
		if(s[i] == '#'){
			int ldist;
			int rdist;

			for(ldist=i-1;ldist>=0 && s[ldist]=='#';ldist--);
			for(rdist=i+1;rdist<n && s[rdist]=='#';rdist++);

			ldist = i - ldist;
			rdist = rdist - i;

			if(maxDist < min(ldist,rdist)){
				res = i;
				maxDist = min(ldist,rdist);
			}
		}
	}

	return res;
}

int main(void){
	int m;

	while(cin>>n>>m && (n||m)){
		memset(s,'#',sizeof(s));

		rep(i,m){
			int idx;
			char ch;
			cin>>ch;

			switch(ch){
			case 'A':
				s[A()] = 'A';
				break;
			case 'B':
				idx = B();
				s[(idx == -1 ? A() : idx)] = 'B';
				break;
			case 'C':
				idx = C();
				s[(idx == -1 ? n%2==0 ? n/2 : n/2 : idx)] = 'C';
				break;
			case 'D':
				s[(i == 0 ? 0 : D())] = 'D';
				break;
			}
		}

		rep(i,n) cout<<s[i];
		cout<<endl;
	}

	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int n,m;
char chair[200];

void A(char ch){
  int i=1;
  while(chair[i]!='#') i++;
  chair[i]=ch;
}

void B(){
  int i=n;
  while(chair[i]!='#')i--;
  chair[i]='B';
}

void C(int kai){
  if(kai==0){
    if(n%2)chair[(n+1)/2]='C';
    else chair[n/2+1]='C';
    return;
  }

  for(int i=1;i<=n;i++){
    if(chair[i]=='#')continue;
    if(chair[i+1]=='#') chair[i+1]='C',i=n+2;
    else if(chair[i-1]=='#') chair[i-1]='C',i=n+2;
   
  }
}

void D(){
  int R=0,L=0;
  while(chair[n-R]=='#')R++;
  while(chair[L+1]=='#')L++;
  
  int M=0,idx=0,cnt=0;
  for(int i=1;i<=n;i++){
    if(chair[i]=='#')cnt++;
    else cnt=0;
    if(M+M%2<cnt) M=cnt,idx=i;
  }
  if(M<=2&&L<=1&&R<=1) A('D');
  else if(L-1>=(M-1)/2&&L-1>=R-1)chair[1]='D';
  else if(R-1>(M-1)/2)chair[n]='D';
  else chair[(idx-M+1)+(M-1)/2]='D';

}

int main(){
  while(1){
    cin>>n>>m;
    if(!n&&!m)break;
    memset(chair,'#',sizeof(chair));
    chair[0]=chair[n+1]=0;
    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();
    }
    printf("%s\n",chair+1);
  }
  return 0;
}