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>
using namespace std;

int main(){

  int n,m;
  int j;
  char chair[100];
  char p;
  int left,max,m_left;

  while(1){

    cin >> n >> m;
    if(n==0 && m==0)
      break;

    for(int i=0; i<n; i++)
      chair[i]='#';


    for(int i=0; i<m; i++){

    cin >> p;

    if(p=='A'){
      j=0;
      while(chair[j]!='#')
	j++;

      chair[j]='A';
    }///////////////////////A

    else if(p=='B'){
      j=n-1;
      while(1){
	if(chair[j]=='#'){
	  if(j==n-1 && chair[j-1]!='A')
	    break;

	  else if(j>0 && chair[j+1]!='A' && chair[j-1]!='A')
	    break;

	  else if(chair[j+1]!='A')
	    break;
	  
	  else if(j==0){
	    j=-1;
	    break;
	  }
	}
	j--;
      }

      if(j!=-1)
	chair[j]='B';

      else{
	j=0;
	while(chair[j]!='#')
	  j++;

	chair[j]='B';
      }

    }////////////////////////B


    else if(p=='C'){
      j=0;
      while(1){
	if(chair[j]!='#'){
	  if(chair[j+1]=='#'&&j!=n-1){
	    j++;
	    break;
	  }
	  else if(j!=0 && chair[j-1]=='#'){
	    j--;
	    break;
	  }
	}

	if(j==n-1){
	  j=-1;
	  break;
	}

	j++;
      }

      if(j==-1){
	if(n%2==1)
	  j=(n+1)/2-1;
	else
	  j=n/2;
      }

      chair[j] = 'C';
    }///////////////////////C


    else{
      left=0; max=1; m_left=0;
      j=1;
      while(j<=n-1){
	if(chair[j]!='#'){
	  left=j;

	  j++;
	}

	if(max < j-left){
	  max = j-left;
	  m_left = left;
	}

	j++;
      }

      if(max <= 2){
	j=0;
	while(j<n && chair[j] != '#')
	  j++;
      }

      else if(max == n-1)
	j=0;

      else{
	if(j==n)
	  j--;
	else if(j==0)
	  continue;
	else if(max%2==1)
	  j=m_left+max/2 +1;
	else
	  j=m_left+max/2;
      }


      chair[j]='D';

    }///////////////////////D

    }


    for(int i=0; i<n; i++)
      cout << chair[i];


    cout << endl;

  }


  return 0;

}

C++: #include<iostream>
using namespace std;
#define REP(i,b,n) for(int i=b;i<n;i++)
#define rep(i,n)   REP(i,0,n)

int findlm(int n,char *in){
  rep(i,n)if(in[i] == 0)return i;
}

int findNotA(int n,char *in){
  for(int i=n-1;i>=0;i--){
    if (in[i] == 0 && (i==0||in[i-1]!= 'A') && (i == n-1 || in[i+1] != 'A'))return i;
  }
  return -1;
}

int nextP(int n,char *in){
  rep(i,n){
    if (in[i] != 0){
      if (i < n-1 && in[i+1] == 0)return i+1;
      if (i >0 && in[i-1] == 0)return i-1;
    }
  }
  return -1;
}

int notNextP(int n,char *in){
  int ret = -1,maxdist=0;
  rep(i,n){
    if( in[i] != 0)continue;
    int tmpdist = 0;
    for(int j=i-1;j>=0;j--){
      if (in[j] != 0){tmpdist=i-j;break;}
    }
    for(int j=i+1;j<n;j++){
      if (in[j] != 0){tmpdist=min(tmpdist,j-i);break;}
    }
    if (tmpdist > maxdist)ret=i,maxdist = tmpdist;
  }
  return ret;
}

void put(int n,char *in,char inp){
  int index = -1;
  if (inp == 'A'){
    index = findlm(n,in);
  }else if (inp == 'B'){
    index = findNotA(n,in);
    if (index == -1)index = findlm(n,in);
  }else if (inp == 'C'){
    index = nextP(n,in);
    if (index == -1)index =n/2;
  }else if (inp == 'D'){
    index = notNextP(n,in);
    if (index == -1)index = 0;
  }
  in[index] = inp;
}


main(){
  char op[255];
  op['A']='A';  op['B']='B';  op['C']='C';  op['D']='D';
  op[0]='#';

  int n,m;
  char in[100];
  while(cin>>n>>m && n){
    rep(i,n)in[i] = 0;
    rep(i,m){
      char inp;
      cin>>inp;
      put(n,in,inp);
    }
    rep(i,n)cout << op[in[i]] ;
    cout << endl;
  }


}

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;
			}
		}
		--i;
	}
	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;
			}
		}
		++i;
	}
	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 ) / 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;
}