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[:len(chair)-1], ""))
	}
}

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 "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';
}