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 <iostream>
#include <vector>
using namespace std;

const int INF = 1e+8;

// s[k] から座っている人までの距離
int dist(vector<char>& s, int k){
	int a=0, b=0;
	for(int i=k ; i >= 0 ; i-- , a++ ){
		if( i == 0 && s[i] == '#' ){
			a = INF;
			break;
		}
		if( s[i] != '#' ) break;
	}
	for(int i=k ; i < s.size() ; i++ , b++ ){
		if( i+1 == s.size() && s[i] == '#' ){
			b = INF;
			break;
		}
		if( s[i] != '#' ) break;
	}
	return min(a-1,b-1);
}

void f(vector<char>& s , char a ){
	if( a == 'A' ){
		// 一番最初に見つかった空席に座る
		for(int i=0 ; i < s.size() ; i++ ){
			if( s[i] == '#' ){
				s[i] = 'A';
				return ;
			}
		}
	}else if( a == 'B' ){
		// 隣が'A'でない空席に座る
		for(int i=s.size()-1 ; i >= 0  ; i-- ){
			if( s[i] != '#' ) continue;
			
			if( i == 0 ){
				if( i+1 == s.size() || s[i+1] != 'A' ){
					s[i] = 'B';
					return;
				}
			}else if( i == s.size()-1 ){
				if( i-1 < 0 || s[i-1] != 'A' ){
					s[i] = 'B';
					return;
				}
			}else{
				if( s[i-1] != 'A' && s[i+1] != 'A' ){
					s[i] = 'B';
					return;
				}
			}
		}
		// 座れる場所がなかった場合一番最初の空席に座る
		for(int i=0 ; i < s.size() ; i++ ){
			if( s[i] == '#' ){
				s[i] = 'B';
				return;
			}
		}
	}else if( a == 'C' ){
		// 左に人がいる空席に座る.
		for(int i=1 ; i < s.size() ; i++ ){
			if( s[i-1] != '#' && s[i] == '#' ){
				s[i] = 'C';
				return;
			}
		}
		// 全部空席のときは真ん中に座る.
		s[s.size()/2] = 'C';
		return;
	}else if( a == 'D' ){
		// 空席の数をチェック.
		int cnt = 0;
		for(int i=0 ; i < s.size() ; i++ ){
			if( s[i] == '#' ) cnt++;
		}
		if( cnt == s.size() ){ // 全部空席のとき
			s[0] = 'D';
			return;
		}
		
		// 一番距離の遠い席を探す.
		int m = dist(s,0);
		for(int i=1 ; i < s.size() ; i++ ){
			m = max( m , dist(s,i) );
		}
		for(int i=0 ; i < s.size() ; i++ ){
			if( m == dist(s,i) ){
				s[i] = 'D';
				return;
			}
		}
	}
}

int main(){
	int n, m;
	while( cin >> n >> m , n || m ){
		vector<char> s(n,'#');
		for(int i=0 ; i < m ; i++ ){
			char a;
			cin >> a;
			f( s , a );
		}
		for(int i=0 ; i < s.size() ; i++ ){
			cout << s[i];
		}
		cout << endl;
	}
}

C++: #include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
//#include <set>

int n, m; //??????, ???

int main() {
    while(std::cin >> n >> m, n + m) {
        std::string str(n, '#');

        char q;
        for(int a = 0; a < m; ++a) {
            std::cin >> q;

            bool ok;
            switch(q) {
            case 'A':
                str[str.find("#")] = 'A';
                break;
            case 'B':
                ok = false;
                for(int i = n - 1; i >= 0; --i) {
                    if((i == 0 || str[i - 1] != 'A') && (i == n - 1 || str[i + 1] != 'A')) {
                        ok = true;
                        str[i] = 'B';
                        break;
                    }
                }
                if(!ok) {
                    str[str.find("#")] = 'B';
                }
                break;
            case 'C':
                ok = false;
                for(int i = 0; i < n; ++i) {
                    if(str[i] != '#') {
                        if(i != n - 1 && str[i + 1] == '#') {
                            str[i + 1] = 'C';
                            ok = true; break;
                        }
                        if(i != 0) {
                            str[i - 1] = 'C';
                            ok = true; break;
                        }
                    }
                }

                if(!ok) {
                    str[n / 2] = 'C';
                }
                break;
            case 'D':
                int dist[101];
                for(int i = 0; i < n; ++i) {
                    int d = 1e+9;
                    if(str[i] == '#') {
                        for(int j = 0; j < n; ++j) {
                            if(str[j] != '#') {
                                d = std::min(d, std::abs(i - j));
                            }
                        }
                    }else {
                        d = 0;
                    }
                    
                    dist[i] = d;
                }

                int maxIdx = 0;
                for(int i = 1; i < n; ++i) {
                    if(dist[i] > dist[maxIdx]) {
                        maxIdx = i;
                    }
                }

                str[maxIdx] = 'D';

                break;
            }
        }

        std::cout << str << std::endl;
    }
}