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 <bits/stdc++.h>
using namespace std;
int n,m;
char chair[200];

void A(char ch){
  int i=1;
  while(chair[i]!='#') i++;
  chair[i]=ch;
  //assert(i<=n);
}

void B(){
  int i=n;
  while(chair[i]!='#')i--;
  chair[i]='B';
  assert(i>=0);
}

void C(int kai){
  if(kai==0){
    if(n%2)chair[(n+1)/2]='C';
    else chair[n/2+1]='C';
    return;
  }
  int f=1;
  for(int i=1;i<=n;i++){
    if(chair[i]=='#')continue;
    if(chair[i-1]=='#') chair[i-1]='C',i=n,f=0;
    if(chair[i+1]=='#') chair[i+1]='C',i=n,f=0;
  }
  assert(f);
}

void D(){
  int R=0,L=0;
  while(chair[n-R]=='#')R++;
  while(chair[L+1]=='#')L++;
  
  int M=0,idx=0,cnt=0;
  for(int i=1;i<=n;i++){
    if(chair[i]=='#')cnt++;
    else cnt=0;
    if(M+M%2<cnt) M=cnt,idx=i;
  }
  if(M<=2&&L<=1&&R<=1) A('D');
  else if(L-1>=(M-1)/2&&L-1>=R-1)chair[1]='D';
  else if(R-1>(M-1)/2)chair[n]='D';
  else chair[(idx-M+1)+(M-1)/2]='D';

}

int main(){
  while(1){
    cin>>n>>m;
    if(!n&&!m)break;
    memset(chair,'#',sizeof(chair));
    chair[0]=chair[n+1]=0;
    for(int i=0;i<m;i++){
      char a;
      cin>>a;
      if(a=='A') A(a);
      if(a=='B') B();
      if(a=='C') C(i);
      if(a=='D') D();
    }
    assert(strlen(chair+1)==n);
    printf("%s\n",chair+1);
  }
  return 0;
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<algorithm>
#include<map>
#include<set>
#include<utility>
#include<cmath>
#include<cstring>
#include<queue>
#include<cstdio>
#include<sstream>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb push_back
#define mp make_pair
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
const double EPS=1e-10;
const int inf=1e8;
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pii;
int main(){
	int n,m;
	while(cin>>n>>m,n){
		vector<string>in(n,"");
		rep(q,m){
			string s;cin>>s;
			if(s=="A"){
				rep(i,n)if(in[i]==""){
					in[i]="A";
					break;
				}
			}else if(s=="B"){
				vector<string>tmp=in;
				rep(i,n)if(in[i]=="A"){
					if(i)tmp[i-1]="!";
					if(i!=n-1)tmp[i+1]="!";
				}
				int I=inf;
				for(int i=n-1;i>=0;i--)if(tmp[i]==""){I=i;break;}
				if(I==inf){
					rep(i,n)if(in[i]==""){
						I=i;
						break;
					}
				}
				in[I]="B";
			}else if(s=="C"){
				int I=inf;
				rep(i,n)if(in[i]==""){
					if((i&&in[i-1]!="")||(i!=n-1&&in[i+1]!="")){
						I=i;
						break;
					}
				}
				if(I==inf)I=n/2;
				in[I]="C";
			}else{
				if(!q){
					in[0]="D";
					continue;
				}
				vi t;
				rep(i,n)if(in[i]!="")t.pb(i);
				int mi=inf,mii=0;
				if(t[0]!=0){mi=t[0];mii=0;}
				rep(i,t.size()-1)if(t[i+1]-t[i]>1){
					if(mi>(t[i+1]-t[i])/2){
						mi=(t[i+1]-t[i])/2;
						mii=t[i]+(t[i+1]-t[i])/2;
					}
				}
				if(t[t.size()-1]!=n-1&&mi>n-t[t.size()-1]-1){
					mii=n-1;
				}
				in[mii]="D";
			}
		}
//		rep(i,n)cout<<(in[i]==""?"#":in[i]);
//		cout<<endl;
	cout<<endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;

int main()
{
	int n, m;
	while (scanf("%d %d", &n, &m), n + m){
		string str;
		str += "*";
		for (int i = 0; i < n; i++){
			str += "#";
		}
		for (int i = 0; i < m; i++){
			string t;
			cin >> t;
			if (t == "A"){
				int j = 1;
				while (str[j] != '#'){
					j++;
				}
				str[j] = 'A';
			}
			else if (t == "B"){
				bool f = false;
				for (int j = str.size(); j >= 0; j--){
					if (str[j] == '#' && str[j - 1] != 'A' && str[j + 1] != 'A'){
						str[j] = 'B';
						f = true;
						break;
					}
				}
				if (!f){
					int j = 1;
					while (str[j] != '#'){
						j++;
					}
					str[j] = 'B';
				}
			}
			else if (t == "C"){
				if (i == 0){
					if (n % 2 == 1){
						str[(n + 1) / 2] = 'C';
					}
					else {
						str[n / 2 + 1] = 'C';
					}
				}
				else {
					for (int j = 1; j < str.size(); j++){
						if (str[j] != '#'){
							if (str[j + 1] == '#'){
								str[j + 1] = 'C';
								break;
							}
							else if (str[j - 1] == '#'){
								str[j - 1] = 'C';
								break;
							}
						}
					}
				}
			}
			else {
				if (i == 0){
					str[1] = 'D';
					continue;
				}
				
				int maxd = -1, maxp;
				for (int j = str.size() - 1; j > 0; j--){
					if (str[j] != '#') continue;
					bool f = false, ff = false;
					for (int k = 1; k <= 100 && !f && !ff; k++){
						if (j - k > 0){
							if (str[j - k] != '#'){
								if (k >= maxd){
									maxd = k;
									maxp = j;
									f = true;
								}
							}
						}
						if (j + k < str.size()){
							if (str[j + k] != '#'){
								if (k >= maxd){
									maxd = k;
									maxp = j;
									ff = true;
								}
							}
						}
					}
				}
				str[maxp] = 'D';
			}
		}
		
		string res = str.substr(1);
		cout << res << endl;
	}
	
	return 0;
}