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 <algorithm>

using namespace std;

int main()
{
	for(int n,m;cin>>n>>m,n|m;){
		string s(n,'#');
		while(m--){
			char p; cin>>p;
			if(p=='A'){
				for(int i=0;i<n;i++)
					if(s[i]=='#'){
						s[i]='A';
						break;
					}
			}
			if(p=='B'){
				bool flg=false;
				for(int i=n-1;i>=0;i--){
					if(s[i]=='#' && (i==n-1 || s[i+1]!='A') && (i==0 || s[i-1]!='A')){
						s[i]='B';
						flg=true;
						break;
					}
				}
				if(!flg){
					for(int i=0;i<n;i++)
						if(s[i]=='#'){
							s[i]='B';
							break;
						}
				}
			}
			if(p=='C'){
				if(count(s.begin(),s.end(),'#')==n)
					s[s.size()/2]='C';
				else{
					for(int i=0;i<n;i++){
						if(s[i]=='#')
							continue;
						if(i<n-1 && s[i+1]=='#'){
							s[i+1]='C';
							break;
						}
						if(i>0 && s[i-1]=='#'){
							s[i-1]='C';
							break;
						}
					}
				}
			}
			if(p=='D'){
				if(count(s.begin(),s.end(),'#')==n)
					s[0]='D';
				else{
					int mi=-1,md;
					for(int i=0;i<n;i++){
						if(s[i]!='#')
							continue;
						int d=n;
						for(int j=i-1;j>=0;j--)
							if(s[j]!='#'){
								d=min(d,i-j);
								break;
							}
						for(int j=i+1;j<n;j++)
							if(s[j]!='#'){
								d=min(d,j-i);
								break;
							}
						if(mi==-1 || d<md){
							mi=i;
							md=d;
						}
					}
					s[mi]='D';
				}
			}
		}
		cout<<s<<endl;
	}
	
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>

int main() {
	int n, m;
	const char output[] = { '#', 'A', 'B', 'C', 'D' };
	std::vector<int> seat;
	
	while (std::cin >> n >> m) {
		if (n == 0 && m == 0)
			break;

		seat.resize(n);

		for (int i = 0; i < m; ++i) {
			char who;
			std::cin >> who;

			if (who == 'A') {
				for (int j = 0; j < n; ++j) {
					if (seat[j] == 0) {
						seat[j] = 1;
						break;
					}
				}
			}
			else if (who == 'B') {
				int firstEmpty = 334;
				bool isRightA = false;

				for (int j = n - 1; j >= 0; --j) {
					if (seat[j] == 0) {
						firstEmpty = std::min(firstEmpty, j);
						if (!isRightA && (j == 0 || seat[j - 1] != 1)) {
							seat[j] = 2;
							break;
						}
					}
					isRightA = seat[j] == 1;
				}
			}
			else if (who == 'C') {
				for (int j = 0; j < n; ++j) {
					if (j != 0) {
						if (j != n - 1 && seat[j + 1] == 0) {
							seat[j + 1] = 2;
							break;
						}
						if (j != 0 && seat[j - 1] == 0) {
							seat[j - 1] = 2;
							break;
						}
					}
					if (j == n - 1)
						seat[n / 2] = 2;
				}
			}
			else if (who == 'D') {
				int maxDist = 334;
				int maxIdx = 0;

				int dist = 0;

				int fromTop = 0;
				int toLast = 0;

				for (int j = 0; j < n; ++j) {
					++dist;
					if (j == n - 1)
						toLast = dist;

					if (seat[j] != 0) {
						if (fromTop == 0)
							fromTop = j + 1;

						if (maxDist < dist / 2) {
							maxDist = dist / 2;
							maxIdx = j - dist;
						}
						dist = 0;
					}
				}

				if (seat[n - 1] == 0 && toLast > maxDist && fromTop < toLast)
					seat[n - 1] = 3;
				else if (seat[0] == 0 && fromTop > maxDist && fromTop > toLast)
					seat[0] = 3;
				else
					seat[maxIdx + maxDist] = 3;
			}
		}

		for (int i = 0; i < m; ++i)
			std::cout << output[i];
	}

	return 0;
}

C++: #include<iostream>
using namespace std;
int n,m;
char t[100],ch;
void putA();
void putB();
void putC();
void putD();
int main(){
  while(cin>>n>>m){
    if(n==0&&m==0)break;
    for(int i=0;i<n;i++)t[i]='#';
    for(int i=0;i<m;i++){
      cin>>ch;
      if(ch=='A')putA();
      else if(ch=='B')putB();
      else if(ch=='C')putC();
      else putD();
    }
    for(int i=0;i<n;i++)cout<<t[i];
    cout<<endl;
  }
  return 0;
}
void putA(){
  for(int i=0;i<n;i++){
    if(t[i]!='#')continue;
    t[i]='A';
    return;
  }
}
void putB(){
  for(int i=n-1;i>=0;i--){
    if(t[i]!='#')continue;
    if(i-1>=0&&t[i-1]=='A')continue;
    if(i+1<n&&t[i+1]=='A')continue;
    t[i]='B';
    return;
  }
  for(int i=0;i<n;i++){
    if(t[i]=='#'){
      t[i]='B';
      return;
    }
  }
}
void putC(){
  for(int i=0;i<n;i++){
    if(t[i]=='#')continue;
    if(i+1<n&&t[i+1]=='#'){
      t[i+1]='C';return;
    }else if(i-1>=0&&t[i-1]=='#'){
      t[i-1]='C';return;
    }
  }
  t[n/2]='C';
}
void putD(){
  bool flg=true;
  for(int i=0;i<n;i++)if(t[i]!='#')flg=false;
  if(flg){
    t[0]='D';return;
  }

  int l[100],r[100],m[100];
  l[0]=(t[0]=='#'?100:0);
  for(int i=1;i<n;i++){
    if(t[i]=='#')l[i]=l[i-1]+1;
    else l[i]=0;
  }
  r[n-1]=(t[n-1]=='#'?100:0);
  
  for(int i=n-2;i>=0;i--){
    if(t[i]=='#')r[i]=r[i+1]+1;
    else r[i]=0;
  }
  //for(int i=0;i<n;i++)cout<<t[i];cout<<endl;
  for(int i=0;i<n;i++){
    m[i]=min(l[i],r[i]);
    //cout<<m[i];
  }
  //cout<<endl;

  int pos=-1,maxm=0;
  for(int i=0;i<n;i++){
    if(t[i]!='#')continue;
    if(m[i]>maxm){
      pos=i;
      maxm=m[i];
    }
  }
  if(pos!=-1){
    t[pos]='D';
  }
}