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
			}
		}
		for i := 0; i < n; i++ {
			if i == n-1 {
				fmt.Println(chair[i])
			} else {
				fmt.Printf("%s ", chair[i])
			}
		}
	}
}

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 <vector>
#include <string.h>
using namespace std;

void sit_seat(char c, vector<char>& seat)
{
	if(c == 'A')
	{
		for(int i=0; i<seat.size(); i++)
		{
			if(seat[i] == 0)
			{
				seat[i] = c;
				break;
			}
		}
	}
	else if(c == 'B')
	{
		bool is_sit = false;
		for(int i=seat.size()-1; i>=0; i--)
		{
			if(seat[i] != 0) continue;

			int r = i+1;
			if(r>=0 && r<seat.size())
			{
				if(seat[r] == 'A') continue;
			}

			int l = i-1;
			if(l>=0 && l<seat.size())
			{
				if(seat[l] == 'A') continue;
			}

			seat[i] = c;
			is_sit = true;
			break;
		}

		if(!is_sit)
		{
			for(int i=0; i<seat.size(); i++)
			{
				if(seat[i] == 0) seat[i] = c;
			}
		}
	}
	else if(c == 'C')
	{
		bool is_sit = false;
		for(int i=0; i<seat.size(); i++)
		{
			if(seat[i] != 0) continue;
			int r = i+1;
			if(r>=0 && r<seat.size())
			{
				if(seat[r] != 0)
				{
					seat[i] = c;
					is_sit = true;
					break;
				}
			}

			int l = i-1;
			if(l>=0 && l<seat.size())
			{
				if(seat[l] != 0)
				{
					seat[i] = c;
					is_sit = true;
					break;
				}
			}
		}

		if(!is_sit)
		{
			if(seat.size() % 2 == 0)
			{
				seat[seat.size()/2+1] = c;
			}
			else
			{
				seat[(seat.size()+1)/2] = c;
			}
		}
	}
	else if(c == 'D')
	{
		bool is_people = false;
		for(int i=0; i<seat.size(); i++)
		{
			if(seat[i] != 0)
			{
				is_people = true;
				break;
			}
		}

		bool is_sit = false;
		if(!is_people)
		{
			seat[0] = c;
			is_sit = true;
		}

		if(!is_sit)
		{
			int mmd = 0;
			int p_sit = 0;
			for(int i=0; i<seat.size(); i++)
			{
				if(seat[i] != 0) continue;
				int r = i;
				while( 1 )
				{
					r++;
					if(r<0 || r>=seat.size()) break;
					if(seat[r] != 0) break;
				}
				int rd = r - i;

				int l = i;
				while( 1 )
				{
					l--;
					if(l<0 || l>=seat.size()) break;
					if(seat[l] != 0) break;
				}
				int ld = i - l;

				int d = min(rd, ld);
				if(mmd < d)
				{
					mmd = d;
					p_sit = i;
				}
			}
			
			seat[p_sit] = c;
		}
	}
	else
	{
		cout << "unknown nationality!" << endl;
		return;
	}
}

int main()
{
	int n, np;
	char c;
	while(cin >> n >> np, n|np)
	{
		vector<char> seat(n);
		memset(&seat[0], 0, sizeof(char)*n);

		for(int i=0; i<np; i++)
		{
			cin >> c;
			sit_seat(c, seat);
		}

		for(int i=0; i<n; i++)
		{
			if(seat[i] !=0) printf("%c", seat[i]);
			else printf("#");
		}
		printf("\n");
	}
}

C++: #include <iostream>
#include <string>
#include <vector>
using namespace std;

const int N_MAX = 100;

void sitA( char* seat, int n )
{
	int i = 0;
	while( seat[ i ] != '#' ){
		++i;
	}
	seat[ i ] = 'A';
}

void sitB( char* seat, int n )
{
	int i = n - 1;
	while( i > 0 ){
		if( seat[ i ] == '#' ){
			if( seat[ i - 1 ] != 'A' && seat[ i + 1 ] != 'A' ){
				seat[ i ] = 'B';
				return;
			}
		}
	}
	while( seat[ i ] != '#' ){
		++i;
	}
	seat[ i ] = 'B';
}

void sitC( char* seat, int n )
{
	int i = 0;
	while( i < n ){
		if( seat[ i ] != '#' ){
			if( seat[ i + 1 ] =='#' ){
				seat[ i + 1 ] = 'C';
				return;
			}
			if( i >= 0 && seat[ i - 1 ] == '#' ){
				seat[ i - 1 ] = 'C';
				return;
			}
		}
	}
	seat[ n / 2 ] = 'C';
}

void sitD( char* seat, int n )
{
	int est[ N_MAX ];
	int s, e;
	vector< int > seated;
	for( int i = 0; i < n; ++i ){
		if( seat[ i ] != '#' ){
			seated.push_back( i );
		}
	}
	if( seated.size() == 0 ){
		seat[ 0 ] = 'D';
		return;
	}
	e = seated[ 0 ];
	for( int i = e; i >= 0; --i ){
		est[ i ] = e - i;
	}
	for( int i = 1; i < seated.size(); ++i ){
		s = e;
		e = seated[ i ];
		for( int j = 0; j <= ( e - s - 1 ) / 2; ++j ){
			est[ s + j ] = est[ e - j ] = j;
		}
	}
	for( int i = e; i < n; ++i  ){
		est[ i ] = i - e;
	}
	int estMax = 0, estIdx = 0;
	for( int i = 0; i < n; ++i ){
		if( est[ i ] > estMax ){
			estMax = est[ i ];
			estIdx = i;
		}
	}
	seat[ estIdx ] = 'D'; 
}

void sit( char a, char* seat, int n )
{
	switch( a ){
		case 'A': sitA( seat, n ); break;
		case 'B': sitB( seat, n ); break;
		case 'C': sitC( seat, n ); break;
		case 'D': sitD( seat, n ); break;
	}
}

int main()
{
	int n, m;
	char seat[ N_MAX + 1 ];
	while( cin >> n >> m && ( n || m ) ){
		for( int i = 0; i < n; ++i ){
			seat[ i ] = '#';
		}
		seat[ n ] = '\0';

		for( int i = 0; i < m; ++i ){
			char a;
			cin >> a;
			sit( a, seat, n );
		}
		cout << seat << endl;
	}
	return 0;
}