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>

using namespace std;

int n,m;
string ans;

bool isEmpty(){
  for(int i = 0; i < ans.length(); i++)
    if(ans[i] != '#') return false;

  return true;
}

bool isVacant(int pos){
  if(pos <0 || pos >= ans.length()) return false;

  return ans[pos] == '#';
}

bool isA(int pos){
  if(pos < 0 || pos >= ans.length()) return false;
  return ans[pos] == 'A';
}

int left(int pos){
  int out = 0;
  for(int i = pos; i >= 0; i--){
    out++;
    if(ans[i] != '#') break;
  }
  return out;
}

int right(int pos){
  int out = 0;
  for(int i = pos; i < ans.length(); i++){
    out++;
    if(ans[i] != '#') break;
  }
  return out;
}


void A(){

  for(int i = 0; i < ans.length(); i++)
    if(ans[i] == '#'){
      ans[i] = 'A';
      return;
    }
 
}
void B(){
  for(int i = ans.length()-1; i >= 0; i--){
    if(ans[i] == '#' && !isA(i+1) && !isA(i-1)){
      ans[i] = 'B';
      return;
    }
  }

  for(int i = 0; i < ans.length(); i++)
    if(ans[i] == '#'){
      ans[i] = 'B';
      return;
    }

}
void C(){

  if(isEmpty()){
    if(n%2) ans[(n+1)/2-1] = 'C';
    else ans[n/2] = 'C';
    return;
  }

  for(int i = 0; i < ans.length(); i++)
    if(ans[i] != '#'){
      if(isVacant(i+1)){
	ans[i+1] = 'C';
	return;
      }
      if(isVacant(i-1)){
	ans[i-1] = 'C';
	return;
      }
    }
}

void D(){

  int d = -1,p = -1;

  if(isEmpty()) ans[0] = 'D';
  else{
    for(int i = 0; i < ans.length(); i++){
      int nowdis;
      if(ans[i] == '#') nowdis = min(left(i),right(i));

      if(nowdis > d){
	d = nowdis;
	p = i;
      }
    }
    ans[p] = 'D';
  }
  
}

void init(){
  string tmp = "";  
  for(int i = 0; i < n; i++) tmp += "#";
  ans = tmp;

}
int main(){
  
  while(cin >> n >> m && n+m){
    init();

    for(int i = 0; i < m; i++){
      char c;
      cin >> c;
      if(c == 'A') A();
      if(c == 'B') B();
      if(c == 'C') C();
      if(c == 'D') D();
      //   cout << "test  " << ans << endl;
    }

    cout << ans << endl;
  }
  return 0;
}

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

char c;
int n,m;

char dt[10002];
char *ch=dt+1;

void sit_left(char c)
{
	for(int i=0; i<n; i++)
	if(ch[i]=='#')
	{
		ch[i]=c;
		return;
	}
}

void typeA()
{
	sit_left('A');
}

void typeB()
{
	for(int i=n-1; i>=0; i--)
	if(ch[i]=='#')
	{
		if(ch[i+1]!='A'&&ch[i-1]!='A')
		{
			ch[i]='B';
			return;
		}
	}

	sit_left('B');
}

void typeC()
{
	for(int i=0; i<n; i++)
	if(ch[i]!='#')
	{
		if(ch[i+1]=='#')
		{
			ch[i+1]='C';
			return;
		}
		if(ch[i-1]=='#')
		{
			ch[i-1]='C';
			return;
		}
	}
}

void typeD()
{
	int dl=99999,dr=99999;
	int dist[10000];
	for(int i=0; i<n; i++)
	{
		dist[i] = 99999;
	}
	for(int i=0; i<n; i++)
	{
		dl++; dr++;

		if(ch[  i  ]!='#') { dl=0; }
		if(ch[n-i-1]!='#') { dr=0; }

		dist[  i  ] = min(dist[  i  ],dl);
		dist[n-i-1] = min(dist[n-i-1],dr);
	}
	dl=0;dr=0;
	for(int i=0; i<n; i++)
	{
		if(dist[i]>dr) { dl=i; dr=dist[i]; }	
	}
	ch[dl] = 'D';
}

int main()
{
	while(cin >> n >> m, (n||m))
	{
		for(int i=0; i<n; i++)
		{
			ch[i] = '#';
		}
		ch[-1] = ch[n] = 'X';

		//first
		cin >> c;
		switch(c)
		{
			case 'A': ch[  0]=c; break;
			case 'B': ch[n-1]=c; break;
			case 'C': ch[n/2]=c; break;
			case 'D': ch[  0]=c; break;
		}
		
		while(--m)
		{
			cin >> c;
			switch(c)
			{
				case 'A': typeA(); break;
				case 'B': typeB(); break;
				case 'C': typeC(); break;
				case 'D': typeD(); break;
			}
		}

		for(int i=0; i<n; i++)
		{
			cout << ch[i];
		}
		cout << endl;
	}

	return 0;
}