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>
using namespace std;
int main(){
  while(true){
    int n,m,j;
    cin>>n>>m;
    if(!n&&!m)break;
    char seat[n+2];
    int len[n];
    bool sit = false;
    for(int i=0;i<n+2;i++){
      seat[i] = '#';
    }
    for(int i=0;i<m;i++){
      char p;
      cin>>p;
      if(p=='A'){
	for(j=1;j<n+1;j++){
	  if(seat[j]=='#'){
	    seat[j] = 'A';
	    break;
	  }
	}
	sit = true;
      }
      if(p=='B'){
	for(j=n;j>0;j--){
	  if(seat[j]=='#'&&seat[j-1]!='A'&&seat[j+1]!='A'){
	    seat[j] = 'B';
	    break;
	  }
	}
	if(j<0){
	  for(j=1;j<n+1;j++){
	    if(seat[j]=='#'){
	      seat[j] = 'B';
	      break;
	    }
	  }
	}
	sit = true;
      }
      if(p=='C'){
	if(!sit){
	  seat[n/2+1] = 'C';
	}
	else{
	  for(j=1;j<n;j++){
	    if(seat[j]!='#'){
	      if(seat[j+1]=='#'){
		seat[j+1] = 'C';
		break;
	      }
	      else if(seat[j-1]=='#'){
		seat[j-1] = 'C';
		break;
	      }
	    }
	  }
	}
	sit = true;
      }
      if(p=='D'){
	if(!sit){
	  seat[1] = 'D';
	}
	else{
	  int l=1000;
	  for(int j=1;j<n+1;j++){
	    if(seat[j]!='#')l = 0;
	    else l++;
	    len[j] = l;
	  }
	  l=1000;
	  int lenmax = 0,maxseat;
	  for(int j=n;j>0;j--){
	    if(seat[j]!='#')l = 0;
	    else l++;
	    len[j] = min(len[j],l);
	    if(lenmax<=len[j]){
	      lenmax=len[j];
	      maxseat=j;
	    }
	  }
	  seat[maxseat] = 'D';
	}
	sit = true;
      }
    }
    for(int i=1;i<n+1;i++)
      cout<<seat[i];
    cout<<endl;
  }
  return 0;
}

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 (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(){

  int n,m;
  int j;
  char chair[100];
  char p;
  int left,max,m_left;

  while(1){

    cin >> n >> m;
    if(n==0 && m==0)
      break;

    for(int i=0; i<n; i++)
      chair[i]='#';


    for(int i=0; i<m; i++){

    cin >> p;

    if(p=='A'){
      j=0;
      while(chair[j]!='#')
	j++;

      chair[j]='A';
    }///////////////////////A

    else if(p=='B'){
      j=n-1;
      while(1){
	if(chair[j]=='#'){
	  if(j==n-1 && chair[j-1]!='A')
	    break;

	  else if(j>0 && chair[j+1]!='A' && chair[j-1]!='A')
	    break;

	  else if(chair[j+1]!='A')
	    break;
	  
	  else{
	    j=-1;
	    break;
	  }
	}
	j--;
      }

      if(j!=-1)
	chair[j]='B';

      else{
	j=0;
	while(chair[j]!='#')
	  j++;

	chair[j]='B';
      }

    }////////////////////////B


    else if(p=='C'){
      j=0;
      while(1){
	if(chair[j]!='#'){
	  if(chair[j+1]=='#'&&j!=n-1){
	    j++;
	    break;
	  }
	  else if(j!=0 && chair[j-1]=='#'){
	    j--;
	    break;
	  }
	}

	if(j==n-1){
	  j=-1;
	  break;
	}

	j++;
      }

      if(j==-1){
	if(n%2==1)
	  j=(n+1)/2;
	else
	  j=n/2+1;
      }

      chair[j] = 'C';
    }///////////////////////C


    else{
      left=0; max=1; m_left=0;
      j=1;
      while(j<=n-1){
	if(chair[j]!='#'){
	  left=j;

	  j++;
	}

	if(max < j-left){
	  max = j-left;
	  m_left = left;
	}

	j++;
      }

      if(max == 1)
	j = m_left+1;

      else if(max == n-1)
	j=0;

      else{
	if(max%2==1)
	  j=m_left+max/2 +1;
	else
	  j=m_left+max/2;
      }


      chair[j]='D';

    }///////////////////////D

    }


    for(int i=0; i<n; i++)
      cout << chair[i];


    cout << endl;

  }


  return 0;

}