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<bits/stdc++.h>
using namespace std;

int main()
{
  int n,m;
  char c,isu[200];
 
  while(1){
    cin>>n>>m;
    if(n+m==0)break;
    for(int i=0;i<200;i++)isu[i]='#';

    for(int k=0;k<m;k++){
      cin>>c;
      if(c=='A'){
	for(int i=0;i<n;i++){
	  if(isu[i]=='#'){
	    isu[i]='A';
	    break;
	  }
	}
      }

      if(c=='B'){
	bool flag=false;
	for(int i=n-1;i>=1;i--){
	  if(isu[i-1]!='A' && isu[i]=='#'){
	    isu[i]='B';
	    flag=true;
	    break;
	  }
	}
	if(flag)continue;
	for(int i=0;i<n;i++){
	  if(isu[i]=='#'){
	    isu[i]='B';
	    break;
	  }
	}
      }
      if(c=='C'){
	if(k==0){
	  if(n%2==0){
	    isu[n/2+1]='C';
	  }
	  else isu[(n+1)/2]='C';
	  continue;
	}
        for(int i = 0;i<n-1;i++){
	  if(isu[i]!='#'){
	    if(isu[i+1]=='#'){
	      isu[i+1]='C';
	      break;
	    }
            if(i!=0 && isu[i-1]=='#'){
	      isu[i-1]='C';
	      break;
	    }
	  }
	}
      }
      if(c=='D') {
	if(k == 0){
	  isu[0]='D';
	  continue;
	}
	int ma=0,index;
	for(int i = 0;i < n;i++){
	  if(isu[i] == '#'){
	    for(int j=i+1;j<n;j++){
	      if(isu[j]!='#' ||  j==n-1){
		if(ma<j-i-1){
		  ma=j-i-1;
		  index=i;
		}
		i=j;
		break;
	      }
	    }
	  }
	}
	isu[index+ma/2]='D';
      }
    }
    for(int i=0;i<n;i++)cout<<isu[i];
    cout<<endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

const int INF = (1 << 30) - 1;

int N, M;
char seat[105];

int left_insert()
{
    int k = 1;
    while ( k <= N && seat[k] != '#') {
        k++;
    }
    return ( k );
}

int main()
{

    while ( cin >> N >> M, N || M ) {

        fill_n(seat, 105, '#');

        for ( int i = 0; i < M; i++ ) {
            char c;
            bool flag = false;
            int d[105];
            int k;

            cin >> c;
            fill_n(d, 105, INF);
            switch ( c ) {
                case 'A' :
                    seat[left_insert()] = 'A';
                    break;
                case 'B' :
                    k = N;
                    while ( k > 0 && seat[k] != '#' && seat[k - 1] == 'A' && seat[k + 1] == 'A' ) {
                        k--;
                    }
                    if ( k > 0 ) {
                        seat[k] = 'B';
                    } else {
                        seat[left_insert()] = 'B';
                    }
                    break;
                case 'C' :
                    for ( int i = 1; i <= N; i++ ) {
                        if ( seat[i] != '#' ) {
                            if ( i < N && seat[i + 1] == '#' ) {
                                seat[i + 1] = 'C';
                                flag = true;
                                break;
                            } else if ( i > 1 && seat[i - 1] == '#' ) {
                                seat[i - 1] = 'C';
                                flag = true;
                                break;
                            }
                        }
                    }
                    if ( !flag ) {
                        seat[N / 2 + 1] = 'C';
                    }
                    break;
                case 'D' :
                    for ( int i = 1; i <= N; i++ ) {
                        if ( seat[i] != '#' ) d[i] = 0;
                        d[i] = min(d[i - 1] + 1, d[i]);
                    }
                    for ( int i = N; i > 0; i-- ) {
                        d[i] = min(d[i], d[i + 1] + 1);
                    }
                    int maxp = 1;
                    for ( int i = 2; i <= N; i++ ) {
                        if ( d[i] > d[maxp] ) {
                            maxp = i;
                        }
                    }
                    seat[maxp] = 'D';
                    break;
            }
        }
        for ( int i = 1; i <= N; i++ ) {
            cout << seat[i];
        }
        cout << endl;
    }
}

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'){
				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;
}