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++: #define _CRT_SECURE_NO_WARNINGS
#include<cstdio>
#include<cmath>
#include<sstream>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<utility>
#include<iomanip>
#include<vector>
#include<string>
#include<queue>
#include<stack>
#include<map>
#include<list>
#include<set>
#include<stdlib.h>
#include<time.h>
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define repp(i,k,n) for(int i=k;i<(int)n;i++)
using namespace std;
const int dx[4] = { 0, 1, 0, -1 }, dy[4] = { 1, 0, -1, 0 };
const int dX[8] = { 0, 1, 1, 1, 0, -1, -1, -1 }, dY[8] = { 1, 1, 0, -1, -1, -1, 0, 1 };
struct XY
{
	int x;
	int y;
};
vector<long long> Prime;
void SieveOfEratosthenes(long long e) //setup vector<long long>Prime(~x)
{
	bool *x=new bool[e];
	for (long long i = 0; i <= e; i++)x[i] = false;
	for (long long i = 2; i <= e; i++)
	{
		if (x[i]);
		else
		{
			Prime.push_back(i);
			for (long long l = i; l <= e; l += i)x[l] = true;
		}
	}
	return;
}
int StringToInt(string a)
{
	stringstream ss;
	int x;
	ss << a;
	ss >> x;
	return x;
}
string IntToString(int a)
{
	stringstream ss;
	string x;
	ss << a;
	ss >> x;
	return x;
}
int GetRandom(int MIN, int MAX)
{
	static int flag=0;
	if (flag == 0)
	{
		srand((unsigned int)time(NULL));
		flag = 1;
	}
	return MIN + (int)(rand()*(MAX - MIN + 1.0) / (1.0 + RAND_MAX));
}
int in()
{
	int x;
	cin >> x;
	return x;
}
void outl(string a)
{
	cout << a << endl;
	return;
}
void outl(int a)
{
	cout << a << endl;
	return;
}
void out(string a)
{
	cout << a;
	return;
}
void out(int a)
{
	cout << a;
	return;
}
/*
(char)'1' - (int)48 = (int)1;
???1111181
 (char)A=(int)65
 (char)a=(int)97
 */
////////////////////////////////////////////////////

int main()
{
	int n, m;
	while (cin >> n >> m&&n&&m)
	{
		vector<char> c;
		rep(i, n)c.push_back('#');
		rep(i, m)
		{
			char now;
			cin >> now;
			bool wan = false;
			bool ok = true;
			switch (now)
			{
			case 'A':
				for (int l = 0;l<c.size(); l++)
				{
					if (c[l] == '#')
					{
						c[l] = 'A';
						break;
					}

				}
				break;
			case'B':
				for (int l = c.size() - 1; l >= 0; l--)
				{
					if (c[l] == '#'&&c[max(0, l - 1)] != 'A'&&c[min(l + 1, (int)c.size() - 1)])
					{
						ok = false;
						c[l] = 'B';
						break;
					}
				}
				if (ok)
				{
					for (int l = 0;l<c.size(); l++)
					{
						if (c[l] == '#')
						{
							c[l] = 'B';
							break;
						}
					}
				}
				break;
			case'C':
				for (int l = 0; l < c.size(); l++)
				{
					if (c[l] != '#')
					{
						if (l-1>=0&&c[l - 1] == '#')
						{
							c[l - 1] = 'C';
							ok = false;
							break;
						}
						else if (l + 1 < c.size() && c[l + 1] == '#')
						{
							c[l + 1] = 'C';
							ok = false;
							break;
						}
					}
				}
				
				if (ok)
				{
					int j = c.size() / 2;
					if (c.size() % 2 == 1)j--;
					c[j] = 'C';
				}
				break;
			case'D':
				if (i == 0)
				{
					c[0] = 'D';
				}
				else
				{
					int ann = 9999;
					int hhh = -1;
					for (int l = c.size() - 1; l >= 0; l--)
					{
						if (c[l] == '#')
						{
							int pp = 0;
							int PP = 0;
							for (int b = l; b >= 0; b--)
							{
								pp++;
								if (c[b] != '#')
								{
									break;
								}
								if (b == 0)pp = 999;
							}

							for (int b = l; b < c.size(); b++)
							{
								PP++;
								if (c[b] != '#')
								{
									break;
								}
								if (b == c.size()-1)PP = 999;
							}
							pp = min(pp, PP);
							if (pp <= ann)
							{
								ann = pp;
								hhh = l;
							}
						}
					}

					c[hhh] = 'D';

				}
				break;
			}
		}

		for (int i = 0; i < c.size(); i++)
		{
			cout << c[i];
		}
		cout << endl;

	}
}

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]] == 'A') {
        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;
}