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;

const int INF = (1<<20);
int n,m;
string ans;

bool isEmpty(){
  for(int i = 0; i < ans.length(); i++)
    if(ans[i] != '#') return false;

  return true;
}

bool isVacant(int pos){
  if(pos <0 || pos >= ans.length()) return false;

  return ans[pos] == '#';
}

bool isA(int pos){
  if(pos < 0 || pos >= ans.length()) return false;
  return ans[pos] == 'A';
}

int left(int pos){
  int out = 0;
  for(int i = pos; i >= 0; i--){
    out++;
    if(ans[i] != '#') return out;
  }
  return INF;
}

int right(int pos){
  int out = 0;
  for(int i = pos; i < ans.length(); i++){
    out++;
    if(ans[i] != '#') return out;
  }
  return INF;
}


void A(){

  for(int i = 0; i < ans.length(); i++)
    if(ans[i] == '#'){
      ans[i] = 'A';
      return;
    }
}

void B(){
  for(int i = ans.length()-1; i >= 0; i--){
    if(ans[i] == '#' && !isA(i+1) && !isA(i-1)){
      ans[i] = 'B';
      return;
    }
  }

  for(int i = 0; i < ans.length(); i++)
    if(ans[i] == '#'){
      ans[i] = 'B';
      return;
    }

}
void C(){

  if(isEmpty()){
    if(n%2) ans[(n+1)/2-1] = 'C';
    else ans[n/2] = 'C';
    return;
  }

  for(int i = 0; i < ans.length(); i++)
    if(ans[i] != '#'){
      if(isVacant(i+1)){
	ans[i+1] = 'C';
	return;
      }
      if(isVacant(i-1)){
	ans[i-1] = 'C';
	return;
      }
    }
}

void D(){

  int d = -1,p = -1;

  if(ans[0] == '#') ans[0] = 'D';
  else{
    for(int i = 0; i < ans.length(); i++){
      int nowdis;
      if(ans[i] == '#') nowdis = min(left(i),right(i));

      if(nowdis > d){
	d = nowdis;
	p = i;
      }
    }
    ans[p] = 'D';
  }
  
}

void init(){
  string tmp = "";  
  for(int i = 0; i < n; i++) tmp += "#";
  ans = tmp;

}
int main(){
  
  while(cin >> n >> m && n+m){
    init();

    for(int i = 0; i < m; i++){
      char c;
      cin >> c;
      if(c == 'A') A();
      if(c == 'B') B();
      if(c == 'C') C();
      if(c == 'D') D();
      //   cout << "test  " << ans << endl;
    }

    cout << ans << endl;
  }
  return 0;
}

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

int main()
{
  int n, m;
  while (cin >> n >> m) {
    string chair(n, '#');

    while (m--) {
      string a;
      cin >> a;
      if (a == "A") {
	for (int i = 0; i < n; ++i) {
	  if (chair[i] == '#') {
	    chair[i] = 'A';
	    break;
	  }
	}
      } else if (a == "B") {
	int sit = -1;
	for (int i = n-1; i > 0 && sit == -1; --i) {
	  if (chair[i] != '#')
	    continue;

	  if (i == 0) {
	    if (chair[i+1] != 'A')
	      sit = i;
	  } else if (i == n-1) {
	    if (chair[i-1] != 'A')
	      sit = i;
	  } else if (chair[i+1] != 'A' && chair[i-1] != 'A') {
	    sit = i;
	  }
	}
	if (sit != -1) {
	  chair[sit] = 'B';
	  continue;
	}

	for (int i = 0; i < n; ++i) {
	  if (chair[i] == '#') {
	    chair[i] = 'B';
	    break;
	  }
	}
      } else if (a  == "C") {
	int sit = -1;
	for (int i = 0; i < n && sit == -1; ++i) {
	  if (chair[i] == '#')
	    continue;

	  if (i == 0) {
	    if (chair[i+1] == '#')
	      sit = i+1;
	  } else if (i == n-1) {
	    if (chair[i-1] == '#')
	      sit = i-1;
	  } else if (chair[i+1] == '#') {
	    sit = i+1;
	  } else if (chair[i-1] == '#') {
	    sit = i-1;
	  }
	}
	if (sit != -1) {
	  chair[sit] = 'C';
	  continue;
	}

	if (n&1)
	  chair[(n+1)/2-1] = 'C';
	else
	  chair[n/2] = 'C';
      } else {
	int maxIndex = -1, dist = -1;
	for (int i = 0; i < n; ++i) {
	  if (chair[i] != '#')
	    continue;

	  int right = 0;
	  if (i == n-1)
	    right = n;
	  for (int j = i+1; j < n; ++j) {
	    if (chair[j] == '#')
	      ++right;
	    else
	      break;
	  }

	  int left = 0;
	  if (i == 0)
	    left = n;
	  for (int j = i-1; j >= 0; --j) {
	    if (chair[j] == '#')
	      ++left;
	    else
	      break;
	  }

	  if (dist < min(right, left)) {
	    maxIndex = i;
	    dist = min(right, left);
	  }
	}
	chair[maxIndex] = 'D';
      }
    }

    cout << chair << endl;
  }
  return 0;
}

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 dmin=100,imin;
	for(int i=0;i<n;i++){
		if(seat[i]=='#'){
			int disL=0,disR=0;
			for(int j=i;j>=0&&seat[j]!='#';j--,disL++);
			for(int j=i;j<n &&seat[j]!='#';j++,disR++);
			int dis=min(disL,disR);
			if(dis<dmin)	dmin=dis,imin=i;
		}
	}
	seat[imin]='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;
}