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
		}
		chairs := make([]string, n)
		for i := range chairs {
			chairs[i] = "#"
		}
		sit := func(who string) {
			if who == "A" {
				chairs[getStringIndex(chairs, "#")] = "A" // A国の人は左から見て行って空いている席を見つけ次第座る
				// log.Println(chairs)
			} else if who == "B" {
				f := true
				for i := n-1; i > -1; i-- {
					// B国の人はA国人の隣以外で、右端から空いている椅子に座る
					if chairs[i] == "#" && (i == 0 || chairs[i-1] != "A") && (i == n-1 || chairs[i+1] != "A") {
								chairs[i] = "B"
								f = false
								break
					}
				}
				// A国人の隣しか空いていない場合、我慢して左端から空いている椅子に座る
				if f {
					chairs[getStringIndex(chairs, "#")] = "B"
				}
				// log.Println(chairs)
			} else if who == "C" { // C国人は人の隣に座りたい
				for i := 0; i < n; i++ {
					if chairs[i] != "#" {
						if i + 1 <= n-1 && chairs[i+1] == "#" {
							chairs[i+1] = "C"
							break
						} else if i - 1 >= 0 && chairs[i-1] == "#" {
							chairs[i-1] = "C"
							break
						}
					}
				}
				// log.Println(chairs)
			} else { // who == "D"
				// D国の人は人から離れて座りたい
				// どうしても誰かの隣に座らなければならない場合、その中で一番左側にある椅子に座る
				scores := make([]int, n)
				for i := range scores {
					scores[i] = INF
				}
				for i, v := range chairs {
					if v == "#" {
						scores[i]++
					}
				}
				for i := n-1; i > -1; i-- {
					tmp := 0
					if chairs[i] == "#" {
						tmp++
					} else {
						tmp = 0
					}
					scores[i] = min(scores[i], tmp)
				}
				chairs[getIntIndex(scores, getMax(scores))] = "D"
				// log.Println(chairs)
			}
		}
		scanner.Scan()
		if first := scanner.Text(); first == "A" || first == "D" {
			chairs[0] = first
		} else if first == "B" {
			chairs[len(chairs)-1] = "B"
		} else {
			chairs[n/2] = "C"
		}
		// log.Println(chairs)
		for i := 0; i < m-1; i++ {
			scanner.Scan()
			sit(scanner.Text())
		}
		fmt.Println(strings.Join(chairs, ""))
	}
}

func getStringIndex(chairs []string, who string) int {
	for i, v := range chairs {
		if v == who {
			return i
		}
	}
	return -1
}

func getIntIndex(scores []int, target int) int {
	for i, v := range scores {
		if v == target {
			return i
		}
	}
	return -1
}

func min(a, b int) int {
	if a > b {
		return b
	}
	return a
}

func getMax(scores []int) int {
	tmp := -1
	for _, v := range scores {
		if tmp < v {
			tmp = v
		}
	}
	return tmp
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<cstring>
#include<algorithm>
#include<map>
#include<set>
#include<cmath>
#include<cassert>
#include<queue>

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;

int ans[111];

int calc(int x, int n) {
	int l = 0;
	for (; l <= x; l++) {
		if (ans[x-l] != -1) break;
	}
	if (l == x+1) l = n;
	int r = 0;
	for (; r+x < n; r++) {
		if (ans[r+x] != -1) break;
	}
	if (r+x == n) r = n;
	return min(l, r);
}

int main() {
	int N, M;
	while (cin >> N >> M) {
		if (N == 0 && M == 0) break;
		memset(ans, -1, sizeof(ans));
		for (int t = 0; t < M; t++) {
			char c;
			cin >> c;
			if (c == 'A') {
				for (int i = 0; i < N; i++) {
					if (ans[i] == -1) {
						ans[i] = 0;
						break;
					}
				}
			}
			if (c == 'B') {
				bool done = false;
				for (int i = N-1; i >= 0; i--) if (ans[i] == -1) {
					if (ans[i+1] != 0) {
						if (i == 0 || ans[i-1] != 0) {
							done = true;
							ans[i] = 1;
							break;
						}
					}
				}
				if (!done) {
					for (int i = 0; i < N; i++) {
						if (ans[i] == -1) {
							ans[i] = 1;
							break;
						}
					}
				}
			}
			if (c == 'C') {
				if (t == 0) {
					ans[(N+1)/2] = 2;
				} else {
					for (int i = 0; i < N; i++) {
						if (ans[i] != -1) {
							if (i+1 < N && ans[i+1] == -1) {
								ans[i+1] = 2;
								break;
							} else if (i > 0 && ans[i-1] == -1) {
								ans[i-1] = 2;
								break;
							}
						}
					}
				}
			}
			if (c == 'D') {
				if (t == 0) {
					ans[0] = 3;
				} else {
					int index = 0, maxi = 0;
					for (int i = 0; i < N; i++) if (ans[i] == -1) {
						int tmp = calc(i, N);
						if (tmp > maxi) {
							maxi = tmp;
							index = i;
						}
					}
					ans[index] = 3;
				}
			}
		}
		for (int i = 0; i < N; i++) {
			if (ans[i] == 0) cout << 'A';
			else if (ans[i] == 1) cout << 'B';
			else if (ans[i] == 2) cout << 'C';
			else if (ans[i] == 3) cout << 'D';
			else cout << '#';
		}
		cout << endl;
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
long n,m,i,j,d[101],e[101],k,dd;
char c,a[101];
while(cin>>n){
cin>>m;
if(n==0)break;
for(i=0;i<n;i++)a[i]='#';
for(j=0;j<m;j++){
cin>>c;
if(c=='A'){
for(i=0;i<n;i++)if(a[i]=='#'){a[i]='A';break;}
}else if(c=='B'){
for(i=n-1;i>=0;i--){
if(a[i]!='#')continue;
if(0<i&&i<n-1&&a[i-1]!='A'&&a[i+1]!='A'){a[i]='B';break;}
else if(0<i&&a[i-1]!='A'){a[i]='B';break;}
else if(i<n-1&&a[i+1]!='A'){a[i]='B';break;}
}
if(i<0){for(i=0;i<n;i++)if(a[i]=='#'){a[i]='B';break;}}
}else if(c=='C'){
for(i=0;i<n;i++){
if(a[i]!='#'){
if(i<n-1&&a[i+1]=='#'){a[i+1]='C';break;}
else if(i>0&&a[i-1]=='#'){a[i-1]='C';break;}
}
}
if(i==n)a[n/2]='C';
}else if(c=='D'){
for(i=0;i<n;i++)d[i]=-1;
for(i=0;i<n;i++)if(a[i]!='#')break;
if(i==n){a[0]='D';continue;}
for(;i<n;i++)if(a[i]=='#')d[i]=d[i-1]+1;else d[i]=0;
for(i=0;i<n;i++)e[i]=-1;
for(i=n-1;i>=0;i--)if(a[i]!='#')break;
for(;i>=0;i--)if(a[i]=='#')e[i]=e[i+1]+1;else e[i]=0;
for(i=0;i<n;i++)if(d[i]<e[i])d[i]=e[i];
dd=d[n-1];
for(i=n-1;i>=0;i--)if(dd<=d[i]){dd=d[i];k=i;}
a[k]='D';
}
}
a[n]=0;cout<<a<<endl;
}
return 0;
}

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;
}

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

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

  }
}

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;
}