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<iostream>
#include<string>
#include<algorithm>

using namespace std;

void PersonA(string&);
void PersonB(string&);
void PersonC(string&);
void PersonD(string&);
bool UsingSeat(const char&);

int main(){
  int i, n, m;
  string c;

  while(1){
    cin >> n >> m;
    if(n == 0 && m == 0) break;
    string chair(n, '#');

    for(i=0; i<m; ++i){
      cin >> c;
      if(c == "A") PersonA(chair);
      else if(c == "B") PersonB(chair);
      else if(c == "C") PersonC(chair);
      else if(c == "D") PersonD(chair);
    }
    cout << chair << endl;
  }
  return 0;
}

void PersonA(string& s){
  string::iterator i;
  i = find(s.begin(), s.end(), '#');
  *i = 'A';
}

void PersonB(string& s){
  string::reverse_iterator i, j;
  string::iterator k;
  for(j=s.rbegin(); i!=s.rend(); j=i+1){
    i = find(j, s.rend(), '#');
    if((i == s.rbegin() && *(i+1) != 'A') ||
       (i == s.rend()-1 && *(i-1) != 'A') ||
       (*(i-1) != 'A' && *(i+1) != 'A')){
      *i = 'B';
      return;
    }
  }
  k = find(s.begin(), s.end(), '#');
  *k = 'B';
}

void PersonC(string& s){
  string::iterator i;
  i = find_if(s.begin(), s.end(), UsingSeat);
  if(i == s.end()){
    s[s.size()/2] = 'C';
    return;
  }
  while(i != s.end()){
    if(*i == '#'){
      *i = 'C';
      return;
    }
    ++i;
  }
}

void PersonD(string& s){
  string::iterator i, j, k;
  i = find_if(s.begin(), s.end(), UsingSeat);
  if(i == s.end()){
    s[0] = 'D';
    return;
  }
  int empty, _empty;
  empty = i - s.begin();
  k = s.begin();
  ++i;
  while(1){
    j = find_if(i, s.end(), UsingSeat);
    if(j == s.end()) break;
    _empty = j - i;
    if(_empty%2 == 0 && _empty/2 > empty){
      empty = _empty/2;
      k = i + empty - 1;
    }
    else if(_empty%2 == 1 && _empty/2 + 1 > empty){
      empty = _empty/2 + 1;
      k = i + empty - 1;
    }
    i = j + 1;
  }
  _empty = s.end() - i;
  if(_empty > empty) k = s.end()-1;
  *k = 'D';
}

bool UsingSeat(const char& c){
  if(c == '#') return false;
  else return true;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <cstring>
#include <sstream>
#include <cassert>
#include <list>
using namespace std;
static const double EPS = 1e-10;
typedef long long ll;
typedef pair<int,int> PI;
typedef vector<int> vi;
#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define F first
#define S second

int dx[]={0,1,0,-1},dy[]={1,0,-1,0};

int seki[111];

main(){
  int m,n;
  while(cin>>n>>m,n|m){
    string seat;
    rep(i,n)seat+='#';

    rep(i,m){
      //cout<<seat<<" "<<m-i<<endl;
      char c;
      int pos=0;
      cin>>c;
      switch(c){
      case 'A':
	pos=0;
	while(seat[pos]!='#')++pos;
	seat[pos]='A';
	break;
      case 'B':
	pos=n-1;
	while(true){
	  if(pos<0)break;
	  if(seat[pos]!='#'){
	    --pos;
	    continue;
	  }
	  if(pos && seat[pos-1]=='A'){
	    --pos;
	    continue;
	  }
	  if(pos+1<n && seat[pos+1]=='A'){
	    --pos;
	    continue;
	  }
	  seat[pos]='B';
	  break;
	}
	if(pos==-1){
	  pos=0;
	  while(seat[pos]!='#')++pos;
	  seat[pos]='B';
	}
	break;
      case 'C':
	if(i==0){
	  if(n%2)seat[(n+1)/2-1]='C';
	  else seat[n/2]='C';
	}else{
	  pos=0;
	  while(seat[pos]=='#')++pos;
	  while(true){
	    if(pos+1<n && seat[pos+1]=='#'){
	      seat[pos+1]='C';
	      break;
	    }else if(pos && seat[pos-1]=='#'){
	      seat[pos-1]='C';
	      break;
	    }
	    ++pos;
	  }
	}
	break;
      case 'D':
	if(i==0){
	  seat[0]='D';
	  continue;
	}
	memset(seki,0,sizeof(0));
	int maxt=0,maxpos=0;

	if(seat[0]=='#'){
	  seki[0]=1;
	  maxt=1;
	  maxpos=1;
	}
	pos=1;
	while(pos<n){
	  if(seat[pos]=='#')seki[pos]=seki[pos-1]+1;
	  else seki[pos]=0;
	  if(seki[pos]>maxt){
	    maxt=seki[pos];
	    maxpos=pos;
	  }
	  ++pos;
	}
	//rep(j,n)cout<<seki[j]<<" ";
	//cout<<endl;
	//cout<<maxpos<<" "<<maxt<<endl;
	if(maxpos==n-1)seat[n-1]='D';
	else if(maxpos==maxt-1)seat[0]='D';
	else if(maxt==2){
	  int pos=0;
	  while(seat[pos]!='#')++pos;
	  seat[pos]='D';
	}else if(maxt%2)seat[maxpos-maxt/2]='D';
	else seat[maxpos-maxt/2]='D';
	break;
      }
    }
    cout<<seat<<endl;
  }
}