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
		}
		chair := make([]string, n+1)
		for i := range chair {
			chair[i] = "#"
		}
		chair[n] = "Sentinel"
		for i := 0; i < m; i++ {
			scanner.Scan()
			switch who := scanner.Text(); who {
			case "A": // 左から眺めて行って空いている席があったら即座に座る
				for j := 0; j < n; j++ {
					if chair[j] == "#" {
						chair[j] = "A"
						break
					}
				}
				// log.Println(chair)
				break // 次の人を読み込む
			case "B":
				flag := true
				// 右端からAの隣でない席を探していく
				for j := n-1; j > -1; j-- {
					if chair[j] == "#" && (j == 0 || chair[j-1] != "A") && chair[j+1] != "A" {
						flag = false
						chair[j] = "B"
						break
					}
				}
				// Aの隣しか空いていないので一番左端の空いている席に座る
				if flag {
					for j := 0; j < n; j++ {
						if chair[j] == "#" {
							chair[j] = "B"
							break
						}
					}
				}
				// log.Println(chair)
				break // 次の人を読み込む
			case "C":
				if i == 0 {
					chair[n/2] = "C" // 先客が誰もいないときは真ん中の椅子に座る
				} else {
					for j := 0; j < n; j++ { // 左から舐めて
						if chair[j] != "#" { // その席が空いていなくて
							if chair[j+1] == "#" { // 右隣が空いているなら座る
								chair[j+1] = "C"
								break
							} else if j != 0 && chair[j-1] == "#" { // 右隣が空いていないので左隣に座る
								chair[j-1] = "C"
								break
							}
						}
					}
				}
				// log.Println(chair)
				break // 次の人を読み込む
			case "D":
				distanceTable := make([]int ,n)
				for j := range distanceTable {
					distanceTable[j] = INF
				}
				for j:= 0; j < n; j++ {
					if chair[j] != "#" {
						for k := 0; k < n; k++ {
							distanceTable[k] = min(distanceTable[k], abs(j-k))
						}
					}
				}
				// log.Println("distanceTAble:", distanceTable)
				at := 0
				distance := 0
				for j := 0; j < n; j++ {
					if distance < distanceTable[j] {
						at = j
						distance = distanceTable[j]
					}
				}
				chair[at] = "D"
				// log.Println(chair)
				break // 次の人を読み込む
			}
		}
		fmt.Println(strings.Join(chair, ""))
	}
}

func min(a, b int) int {
	if a > b {
		return b
	}
	return a
}

func abs(a int) int {
	if a < 0 {
		return -a
	}
	return a
}

C++: #include<iostream>
#include<string.h>
#include<cstdio>
using namespace std;
int main() {
	int n, m, temp;
	char str, str2[101], str3[101] = { NULL };
	while (1) {
		cin >> n >> m;
		if (n == 0 && m == 0)return 0;
		for (int i = 0; i < m; i++) {
			cin >> str2[i];
		}
		for (int i = 0; i < n; i++) {
			str3[i] = '#';
		}
		for (int i = 0; i < m; i++) {
			if (str2[i] == 'A') {
				for (int j = 0; j < n; j++) {
					if (str3[j] == '#') {
						str3[j] = 'A';
						goto end;
					}
				}
			}
			else if (str2[i] == 'B') {
				for (int j = n - 1; j > 0; j--) {
					if (str3[j] == '#'&&str3[j - 1] != 'A'&&str3[j + 1] != 'A') {
						str3[j] = 'B';
						temp++;
						goto end;
					}
				}
				if (temp == 0) {
					for (int j = 0; j < n; j++) {
						if (str3[j] == '#') {
							str3[j] = 'B';
							temp++;
							goto end;
						}
					}
				}
				if (temp == 0) {
					for (int j = 0; j < n; j++) {
						if (str3[j] == '#') {
							str3[j] = 'B';
							goto end;
						}
					}
				}
				temp = 0;
			}
			else if (str2[i] == 'C') {
				for (int j = 0; j < n; j++) {
					if (str3[j] != '#') {
						if (str3[j + 1] == '#') {
							str3[j + 1] = 'C';
							temp++;
							goto end;
						}
					}
				}
				if (temp == 0) {
					if (n % 2 == 0)temp = n / 2 + 1;
					else temp = (n + 1) / 2;
					for (int j = 0; j < n; j++) {
						//if (str3[j] == '#')/*goto end*/;
						if (j == n - 1)str3[temp] = 'C';
					}
				}
				temp = 0;
			}
			else if (str2[i] == 'D') {
				temp = 0;
				int gun = 0, x, y;
				for (int j = 0; j < n; j++) {
					int  q = 0, w = 0;
					if (str3[j] == '#') {
						x = y = j;
						while (1) {
							x++;
							y--;
							if (x <= n || y > 0) {
								if (str3[x] == '#' &&x <= n) {
									q++;
									if (gun < q && gun>0) {
										str3[x] = 'D';
										str3[gun] = '#';
										gun = q;
										temp++;
									}
								}
								if (str3[y] == '#' &&y > 0) {
									w++;
									if (gun < w&& gun>0) {
										str3[y] = 'D';
										str3[gun] = '#';
										gun = w;
										temp++;
									}
								}
							}
							else {
								goto loop;
							}
						}
					}
				}
			loop:
				if (temp == 0) {
					for (int j = 0; j < n; j++) {
						if (str3[j] == '#') {
							str3[j] = 'D';
							goto end;
						}
						if (j == n - 1) {
							str3[0] = 'D';
						}
					}
				}
				temp = 0;
			}
		end:;
			/*for (int i = 0; i < n; i++) {
				cout << str3[i];
			}
			cout << endl;*/
		}
		for (int i = 0; i < n; i++) {
			cout << str3[i];
		}
		cout << endl;
		memset(str3, 0, sizeof(str3));
		memset(str2, 0, sizeof(str2));
		temp = 0;
	}
}

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;
		return;
	}
	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;
	}
	assert(false);
}

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;
}