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"
#include<unordered_map>
#include<unordered_set>
#pragma warning(disable:4996)
using namespace std;
using ld = long double;
const ld eps = 1e-9;

//// < "d:\d_download\visual studio 2015\projects\programing_contest_c++\debug\a.txt" > "d:\d_download\visual studio 2015\projects\programing_contest_c++\debug\b.txt"

void sit(vector<int>&seats, int who) {
	if (who == 1) {
		for (int i = 0; i < seats.size(); ++i) {
			if (!seats[i]) {
				seats[i] = who;
				return;
			}
		}
	}
	else if (who == 2) {
		for (int i = seats.size()-1; i >= 0; --i) {
			if (!seats[i]) {
				bool ok = true;
				for (int k = i+1; k >= i-1; k -= 2) {
					if (k >= 0 && k < seats.size()) {
						if (seats[k] == 1)ok = false;
					}
				}
				if (ok) {
					seats[i] = who;
					return;
				}
			}
		}
		for (int i = 0; i < seats.size(); ++i) {
			if (!seats[i]) {
				seats[i] = who;
				return;
			}
		}
	}
	else if (who == 3) {
		for (int i = 1; i < seats.size(); ++i) {
			if (seats[i - 1]) {
				if (!seats[i]) {
					seats[i] = who;
					return;
				}
			}
		}
		seats[seats.size() / 2] = who;
	}
	else {
		vector<int>diss(seats.size(), 1000);
		for (int i = 0; i < seats.size(); ++i) {
			if (seats[i]) {
				diss[i] = 0;
				for (int j = i - 1; j >= 0; --j) {
					diss[j] = min(diss[j], i-j);
				}
				for (int j = i + 1; j < seats.size(); ++j) {
					diss[j] = min(diss[j], j - i);
				}
			}
		}
		int maxdis = -1;
		int ans = -1;
		for (int i = 0; i < seats.size(); ++i) {
			if (maxdis < diss[i]) {
				maxdis = diss[i];
				ans = i;
			}
		}
		seats[ans] = who;
		return;
	}
}

int main() {
	while (1) {
		int N, M; cin >> N >> M;
		if (!N)break;
		vector<int>seats(N);
		for (int i = 0; i < M; ++i) {
			char c; cin >> c;
			int num = c - 'A' + 1;
			sit(seats, num); 
			for (int i = 0; i < N; ++i) {
				if (seats[i]) {
					cout << char('A' + seats[i] - 1);
				}
				else {
					cout << '#';
				}
			}
			cout << endl;
		}
		
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
void A(vector<char>&);
void B(vector<char>&);
void C(vector<char>&);
void D(vector<char>&);
char get(vector<char>&, bool, int);
int main() {
	int n, m;
	while (cin >> n >> m&&n != 0) {
		vector<char> line(n + 1, '#');
		for (int i = 0; i < m; ++i) {
			char num;
			cin >> num;
			if (num == 'A') {
				A(line);
			}
			else if (num == 'B') {
				B(line);
			}
			else if (num == 'C') {
				C(line);
			}else{
				D(line);
			}
		}
		for (int i = 1; i < line.size(); ++i) {
			cout << line[i];
		}
		cout << endl;
	}
	return 0;
}

char get(vector<char>&line, bool right, int place) {
	if (right) {
		if (place == line.size() - 1)return '#';
		else return line[place + 1];
	}
	else {
		return line[place - 1];
	}
}

void A(vector<char> &line) {
	for (int i = 1; i < line.size(); ++i) {
		if (line[i] == '#') {
			line[i] = 'A';
			break;
		}
	}
}

void B(vector<char> &line) {
	for (int i = line.size() - 1; i > 0; --i) {
		if (get(line, true, i) != 'A' && get(line, false, i) != 'A') {
			line[i] = 'B';
			return;
		}
	}
	for (int i = 1; i < line.size(); ++i) {
		if (line[i] == '#') {
			line[i] = 'B';
			return;
		}
	}
}

void C(vector<char> &line) {
	for (int i = 1; i < line.size(); ++i) {
		if (line[i] != '#') {
			if (i != line.size() - 1 && line[i + 1] == '#') {
				line[i + 1] = 'C';
				return;
			}
			else if (i != 1 && line[i - 1] == '#') {
				line[i - 1] = 'C';
				return;
			}
		}
	}
	if (line.size() % 2 == 0) {
		line[line.size() / 2] = 'C';
		return;
	}
	else {
		line[line.size() / 2 + 1] = 'C';
	}
}

void D(vector<char> &line) {
	vector<int> dline(line.size());
	int num = line.size() + 1;
	bool flag = false;
	for (int i = 1; i < line.size(); ++i) {
		++num;
		if (line[i] != '#') {
			num = 0;
			flag = true;
		}
		dline[i] = num;
	}
	if (!flag) {
		line[1] = 'D';
		return;
	}
	num = line.size() + 1;
	for (int i = line.size() - 1; i > 0; --i) {
		++num;
		if (line[i] != '#') {
			num = 0;
		}
		dline[i] = min(dline[i], num);
	}
	int mn = 0, mp = 0;
	for (int i = 1; i < line.size(); ++i) {
		if (mn < dline[i]) {
			mn = dline[i];
			mp = i;
		}
	}
	line[mp] = 'D';
}