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;

typedef long long ll;
typedef pair<int, int> P;

#define pb push_back
#define rep(i, a, n) for(int i = (a); i < (n); i++)
#define dep(i, a, n) for(int i = (a); i >= (n); i--)
#define mod 1e9+7

__attribute__((constructor))
void initial() {
  cin.tie(0);
  ios::sync_with_stdio(false);
}

int main() {
  int n, m;
  while(cin >> n >> m) {
    if(!n) return 0;
    char a[100] = {};
    rep(j, 0, m) {
      char b;
      cin >> b;
      if(b == 'A') {
        rep(i, 0, n) {
          if(!a[i]) {
            a[i] = b;
            break;
          }
        }
      }else if(b == 'B') {
        dep(i, n - 1, 1) {
          if(!a[i] && a[i - 1] != 'A') a[i] = b;
          break;
        }
        rep(i, 0, n) {
          if(!a[i]) a[i] = b;
          break;
        }
      }else if(b == 'C') {
        int f = 0;
        rep(i, 0, n - 1) {
          if(!a[i]) continue;
          if(i + 1 < n && !a[i + 1]) {
            a[i + 1] = b;
            f = 1;
            break;
          }else if(i - 1 >= 0 && !a[i - 1]) {
            a[i - 1] = b;
            f = 1;
            break;
          }
        }
        if(!f) {
          if(n % 2) a[(n + 1) / 2 - 1] = b;
          else a[n / 2] = b;
        }
      }else {
        int mm = 0, re = 0, an;
        rep(i, 0, n) {
          if(!a[i]) re++;
          else break;
        }
        if(re > 0) {
          an = 0;
          mm = re - 1;
        }
        re = 0;
        dep(i, n - 1, 0) {
          if(!a[i]) re++;
          else break;
        }
        if(mm < re - 1) {
          an = n - 1;
          mm = re - 1;
        }
        re = 0;
        rep(i, 1, n - 1) {
          if(a[i]) continue;
          int l = 0, r = 0;
          dep(k, i - 1, 0) {
            if(!a[k]) l++;
            else break;
          }
          rep(k, i + 1, n) {
            if(!a[k]) r++;
            else break;
          }
          if(mm < min(l, r)) {
            an = i;
            mm = min(l, r);
          }
        }
        if(mm) a[an] = b;
        else {
          rep(i, 0, n) {
            if(!a[i]) {
              a[i] = b;
              break;
            }
          }
        }
      }
    }
    rep(i, 0, n) {
      if(a[i] == 'A' || a[i] == 'B' || a[i] == 'C' || a[i] == 'D') {
        cout << a[i];
      }else cout << '#';
    }
    cout << endl;
  }

}

C++: #include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <cmath>
#include <stack>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <list>
#include <iostream>
#include <sstream>
#include <climits>
#include <cfloat>
#include <complex>

using namespace std;

int main()
{
	int n, m;
	while (scanf("%d %d", &n, &m), n | m)
	{
		char t[102];	// 1~n
		memset(t, '#', sizeof(t));
		for (int w = 0; w < m; ++w)
		{
			char c;
			scanf(" %c", &c);
			if (c == 'A')
			{
				int i;
				for (i = 1; t[i] != '#'; ++i)
					;
				t[i] = 'A';
			}
			else if (c == 'B')
			{
				int i;
				for (i = n; i >= 1; --i)
					if (t[i-1] != 'A' && t[i] == '#' && t[i+1] != 'A')
						break;
				if (i == 0)
					for (i = 1; t[i] != '#'; ++i)
						;
				t[i] = 'B';
			}
			else if (c == 'C')
			{
				int i;
				if (w == 0)
					i = n / 2 + 1;
				else
				{
					for (i = 2; ; ++i)
						if (t[i-1] != '#' && t[i] == '#')
							break;
					if (i == n+1)
						for (--i; t[i] != '#'; --i)
							;
				}
				t[i] = 'C';
			}
			else
			{
				int i;
				if (w == 0)
					i = 1;
				else
				{
					int dis[102];
					for (int j = 0; j < 102; ++j)
						dis[j] = 100000000;
					for (int j = 1; j <= n; ++j)
					{
						if (t[j] != '#')
							for (int k = 1; k <= n; ++k)
								dis[k] = min(dis[k], abs(j-k));
					}
					int d = 0;
					for (int j = 1; j <= n; ++j)
					{
						if (dis[j] > d)
						{
							i = j;
							d = dis[j];
						}
					}
				}
				t[i] = 'D';
			}
		}
		for (int i = 1; i <= n; ++i)
			putchar(t[i]);
		printf("\n");
	}

	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int n,m;
char t[120];

void A(char a){
  int i=1;
  while(t[i]!=0) i++;
  t[i]=a;
}

void B(){
  int i;
  for(i=n;i>=1;i--){
    if(t[i]!=0)continue;
    if(t[i+1]=='A')continue;
    if(t[i-1]=='A')continue;
    t[i]='B';
    break;
  }
  if(i==0) A('B');
}

void C(int cnt){
  if(cnt==0) {
    if(n%2) t[n/2+1]='C';
    else t[(n+1)/2]='C';
    return;
  }

  for(int i=1;i<=n;i++){
    if(t[i]!=0&&t[i+1]==0) {
      t[i+1]='C';
      break;
    }
    if(t[i]!=0&&t[i-1]==0){
      t[i-1]='C';
      break;
    }
  }
}

void D(int cnt){
  if(cnt==0){
    t[1]='D';
    return;
  }
  int mx=0,idx=0,c=0;
  for(int i=1;i<=n;i++) {
    if(t[i]==0) c++;
    else c=0;
    if(mx<c)mx=c,idx=i;
  }
  if(mx<=2){
    A('D');
    return ;
  }

  t[(idx-(idx-mx+1))/2+(idx-mx+1)]='D';
}

int main(){
  while(1){
    cin>>n>>m;
    if(!n&&!m)break;
    memset(t,0,sizeof(t));
    t[0]=t[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(i);
    }

    for(int i=1;i<=n;i++)if(t[i]==0)t[i]='#';
    for(int i=1;i<n;i++) cout <<t[i];
    cout <<t[n]<<endl;
  }
  return 0;
}