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 <sstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <climits>
#include <cfloat>
using namespace std;

int main()
{
    for(;;){
        int n, m;
        cin >> n >> m;
        if(n == 0)
            return 0;

        string s(n, '#');
        for(int i=0; i<m; ++i){
            char c;
            cin >> c;
            if(c == 'A'){
                *find(s.begin(), s.end(), '#') = 'A';
            }else if(c == 'B'){
                bool ok = false;
                for(int j=n-1; j>=0; --j){
                    if(s[j] != '#')
                        continue;
                    if(j+1 < n && s[j+1] == 'A')
                        continue;
                    if(j-1 >= 0 && s[j-1] == 'A')
                        continue;
                    s[j] = 'B';
                    ok = true;
                    break;
                }
                if(!ok)
                    *find(s.begin(), s.end(), '#') = 'B';
            }else if(c == 'C'){
                if(i == 0){
                    s[n/2] = 'C';
                }else{
                    for(int j=0; j<n; ++j){
                        if(s[j] != '#'){
                            if(j+1 < n && s[j+1] == '#'){
                                s[j+1] = 'C';
                                break;
                            }else if(j-1 >= 0 && s[j-1] == '#'){
                                s[j-1] = 'C';
                                break;
                            }
                        }
                    }
                }
            }else{
                if(i == 0){
                    *find(s.begin(), s.end(), '#') = 'D';
                }else{
                    vector<int> dist(n, INT_MAX);
                    for(int j=0; j<n; ++j){
                        if(s[j] == '#')
                            continue;
                        int k = j;
                        do{
                            dist[k] = j - k;
                        }while(--k >= 0 && s[k] == '#');
                        k = j;
                        do{
                            dist[k] = k - j;
                        }while(++ k < n && s[k] == '#');
                    }
                    s[max_element(dist.begin(), dist.end()) - dist.begin()] = 'D';
                }
            }
        }

        cout << s << endl;
    }
}

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

typedef long long ll;
typedef pair<int, int> P;

#define pb push_back
#define rep(i, a, n) for(int i = (a); i < (n); i++)
#define dep(i, a, n) for(int i = (a); i >= (n); i--)
#define mod 1e9+7

__attribute__((constructor))
void initial() {
  cin.tie(0);
  ios::sync_with_stdio(false);
}

int main() {
  int n, m;
  while(cin >> n >> m) {
    if(!n) return 0;
    char a[100] = {};
    rep(j, 0, m) {
      char b;
      cin >> b;
      if(b == 'A') {
        rep(i, 0, n) {
          if(!a[i]) {
            a[i] = b;
            break;
          }
        }
      }else if(b == 'B') {
        dep(i, n - 1, 0) {
          if(a[i]) continue;
          int f = 0;
          if(i + 1 > n) f = 1;
          if(!f && a[i + 1] != 'A') f = 1;
          if(i - 1 < 0 && f) f = 2;
          if(f && a[i - 1] != 'A') f = 2;
          if(f == 2) {
            a[i] = b;
            break;
          }
        }
        rep(i, 0, n) {
          if(!a[i]) a[i] = b;
          break;
        }
      }else if(b == 'C') {
        int f = 0;
        rep(i, 0, n - 1) {
          if(!a[i]) continue;
          if(i + 1 < n && !a[i + 1]) {
            a[i + 1] = b;
            f = 1;
            break;
          }else if(i - 1 >= 0 && !a[i - 1]) {
            a[i - 1] = b;
            f = 1;
            break;
          }
        }
        if(!f) {
          if(n % 2) a[(n + 1) / 2 - 1] = b;
          else a[n / 2] = b;
        }
      }else {
        int mm = 0, re = 0, an;
        rep(i, 0, n) {
          if(!a[i]) re++;
          else break;
        }
        if(re > 0) {
          an = 0;
          mm = re - 1;
        }
        re = 0;
        dep(i, n - 1, 0) {
          if(!a[i]) re++;
          else break;
        }
        if(mm < re - 1) {
          an = n - 1;
          mm = re - 1;
        }
        re = 0;
        rep(i, 1, n - 1) {
          if(a[i]) continue;
          int l = 0, r = 0;
          dep(k, i - 1, 0) {
            if(!a[k]) l++;
            else break;
          }
          rep(k, i + 1, n) {
            if(!a[k]) r++;
            else break;
          }
          if(mm < min(l, r)) {
            an = i;
            mm = min(l, r);
          }
        }
        if(mm) a[an] = b;
        else {
          rep(i, 0, n) {
            if(!a[i]) {
              a[i] = b;
              break;
            }
          }
        }
      }
    }
    rep(i, 0, n) {
      if(a[i] == 'A' || a[i] == 'B' || a[i] == 'C' || a[i] == 'D') {
        cout << a[i];
      }else cout << '#';
    }
    cout << endl;
  }

}