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;
	while (scanf("%d %d", &n, &m), n + m){
		string str;
		str += "*";
		for (int i = 0; i < n; i++){
			str += "#";
		}
		for (int i = 0; i < m; i++){
			string t;
			cin >> t;
			if (t == "A"){
				int j = 1;
				while (str[j] != '#'){
					j++;
				}
				str[j] = 'A';
			}
			else if (t == "B"){
				bool f = false;
				for (int j = str.size(); j >= 0; j--){
					if (str[j] == '#' && str[j - 1] != 'A' && str[j + 1] != 'A'){
						str[j] = 'B';
						f = true;
						break;
					}
				}
				if (!f){
					int j = 1;
					while (str[j] != '#'){
						j++;
					}
					str[j] = 'B';
				}
			}
			else if (t == "C"){
				if (i == 0){
					if (n % 2 == 1){
						str[(n + 1) / 2] = 'C';
					}
					else {
						str[n / 2 + 1] = 'C';
					}
				}
				else {
					for (int j = 1; j < str.size(); j++){
						if (str[j] != '#'){
							if (str[j + 1] == '#'){
								str[j + 1] = 'C';
								break;
							}
							else if (str[j - 1] == '#'){
								str[j - 1] = 'C';
								break;
							}
						}
					}
				}
			}
			else if (t == "D"){
				if (i == 0){
					str[1] = 'D';
					continue;
				}
				
				int maxd = -1, maxp;
				for (int j = 1; j < str.size(); j++){
					if (str[j] != '#') continue;
					bool f = false, ff = false;
					for (int k = 1; k <= 100 && !f; k++){
						if (j - k > 0){
							if (str[j - k] != '#'){
								if (maxd < k){
									maxd = k;
									maxp = j;
								}
								f = true;
								continue;
							}
						}
						if (j + k < str.size()){
							if (str[j + k] != '#'){
								if (maxd < k){
									maxd = k;
									maxp = j;
								}
								f = true;
								continue;
							}
						}
					}
				}
				str[maxp] = 'D';
			}
		}
		
		string res = str.substr(1);
		cout << res << endl;
	}
	
	return 0;
}

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

vector<int> tonari(const string &s, int pos) {
  vector<int> v;
  for(int d = -1; d <= 1; d += 2) {
    int npos = pos + d;
    if(npos < 0 || npos >= s.size()) continue;
    v.push_back(npos);
  }
  return v;
}

int A(const string &s) {
  for(int i = 0; i < s.size(); ++i) {
    if(s[i] == '#') return i;
  }
  return -1;
}

int B(const string &s) {
  for(int i = s.size()-1; i >= 0; --i) {
    vector<int> v = tonari(s, i);
    bool flag = true;
    for(int j = 0; j < v.size(); ++j) {
      if(s[v[j]] != '#') {
        flag = false;
        break;
      }
    }
    if(flag) return i;
  }
  return A(s);
}

int C(const string &s) {
  for(int i = 0; i+1 < s.size(); ++i) {
    if(s[i] != '#' && s[i+1] == '#') return i+1;
  }
  return s.size()/2;
}

int D(const string &s) {
  int maxi = -1, pos = -1;
  for(int i = 0, j = -1; i <= s.size(); ++i) {
    if(i == s.size() || s[i] != '#') {
      if(i-j-1 > maxi) {
        maxi = i-j-1;
        pos = j+1;
      }
      j = i;
    }
  }
  if(maxi == s.size()) return A(s);
  return pos + (maxi-1)/2;
}

int main() {
  for(int n, m; cin >> n >> m && (n|m);) {
    string s(n, '#');
    while(m--) {
      char c;
      cin >> c;
      int pos;
      switch(c) {
      case 'A': pos = A(s); break;
      case 'B': pos = B(s); break;
      case 'C': pos = C(s); break;
      case 'D': pos = D(s); break;
      }
      s[pos] = c;
    }
    cout << s << endl;
  }
  return 0;
}

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

vector<int> tonari(const string &s, int pos) {
  vector<int> v;
  for(int d = -1; d <= 1; d += 2) {
    int npos = pos + d;
    if(npos < 0 || npos >= s.size()) continue;
    v.push_back(npos);
  }
  return v;
}

int A(const string &s) {
  for(int i = 0; i < s.size(); ++i) {
    if(s[i] == '#') return i;
  }
  return -1;
}

int B(const string &s) {
  for(int i = s.size()-1; i >= 0; --i) {
    vector<int> v = tonari(s, i);
    bool flag = true;
    for(int j = 0; j < v.size(); ++j) {
      if(s[v[j]] != '#') {
        flag = false;
        break;
      }
    }
    if(flag) return i;
  }
  return A(s);
}

int C(const string &s) {
  for(int i = 0; i+1 < s.size(); ++i) {
    if(s[i] != '#') {
      if(s[i+1] == '#') return i+1;
      if(i && s[i-1] == '#') return i-1;
    }
  }
  return s.size()/2;
}

int D(const string &s) {
  int maxi = -1, pos = -1;
  for(int i = 0, j = -1; i <= s.size(); ++i) {
    if(i == s.size() || s[i] != '#') {
      if(i-j-1 > maxi) {
        maxi = i-j-1;
        pos = j+1;
      }
      j = i;
    }
  }
  if(maxi == s.size()) return A(s);
  return pos + (maxi-1)/2;
}

int main() {
  for(int n, m; cin >> n >> m && (n|m);) {
    string s(n, '#');
    while(m--) {
      char c;
      cin >> c;
      int pos;
      switch(c) {
      case 'A': pos = A(s); break;
      case 'B': pos = B(s); break;
      case 'C': pos = C(s); break;
      case 'D': pos = D(s); break;
      }
      s[pos] = c;
    }
    cout << s << endl;
  }
  return 0;
}