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<bits/stdc++.h>
using namespace std;

int main(){

  while(1){

    int n,m;

    cin >> n >> m;

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

    char ch[n];

    for(int i=0;i<n;i++){
      ch[i] = '#';
    }

    for(int k=0;k<m;k++){
      char c;

      cin >> c;

      if(c=='A'){
        for(int i=0;i<n;i++){
          if(ch[i]=='#'){
            ch[i] = 'A';
            break;
          }
        }
      }
      else if(c == 'B'){
        bool flag = true;
        for(int i=n-1;i!=0;i--){
          if(ch[i]=='#'){
            if(i == n-1 && ch[i-1]!='A'){
              ch[i] = 'B';
              flag = false;
              break;
            }
            else if(i == 0 && ch[i+1]!='A'){
              ch[i] = 'B';
              flag = false;
              break;
            }
            if((ch[i+1]!='A') && (ch[i-1]!='A')){
              ch[i] = 'B';
              flag = false;
              break;
            }
          }
        }
        if(flag){
          for(int i=0;i<n;i++){
            if(ch[i]=='#'){
              ch[i] = 'B';
              break;
            }
          }
        }
      }
      else if(c == 'C'){
        if(k==0){
          if(n%2==0){
            ch[n/2+1-1] = 'C';
          }
          else{
            ch[(n+1)/2 -1] = 'C';
          }
        }
        else{
          for(int i=0;i<n;i++){
            if(ch[i]!='#'){
              if(ch[i+1]=='#'){
                ch[i+1] = 'C';
                break;
              }
              else if(i!=0 && ch[i-1]=='#'){
                ch[i-1] = 'C';
                break;
              }
              else{
                for(int j=i;j<n;j++){
                  if(ch[j]=='#'){
                    ch[j] = 'C';
                    break;
                  }
                }
              }
            }
          }
        }
      }
      else{
        int dis[n];
        int mdis = 0,pos = 0;
        if(k==0){
          ch[0] = 'D';
        }
        else{
          for(int i=0;i<n;i++){
            if(ch[i]!='#'){
              dis[0] = i;
              break;
            }
          }
          for(int i=1;i<n;i++){
            if(ch[i] != '#'){
              dis[i] = 0;
              continue;
            }
            dis[i] = dis[i-1] + 1;
            for(int j=i+1;j<n;j++){
              if(ch[j] != '#'){
                dis[i] = min(dis[i],j-i);
              }
            }
          }
          for(int i=0;i<n;i++){
            if(dis[i] > mdis){
              mdis = dis[i];
              pos = i;
            }
          }
          ch[pos] = 'D';
        }
      }
    }

    for(int i=0;i<n;i++){
      cout << ch[i];
    }
    cout << endl;

  }
}

C++: #include<iostream>

using namespace std;

int n,m;
string ans;

bool isEmpty(){
  for(int i = 0; i < ans.length(); i++)
    if(ans[i] != '#') return false;

  return true;
}

bool isVacant(int pos){
  if(pos <0 || pos >= ans.length()) return false;

  return ans[pos] == '#';
}

bool isA(int pos){
  if(pos < 0 || pos >= ans.length()) return false;
  return ans[pos] == 'A';
}

int left(int pos){
  int out = 0;
  for(int i = pos; i >= 0; i--){
    out++;
    if(ans[i] != '#') break;
  }
  return out;
}

int right(int pos){
  int out = 0;
  for(int i = pos; i < ans.length(); i++){
    out++;
    if(ans[i] != '#') break;
  }
  return out;
}


void A(){

  for(int i = 0; i < ans.length(); i++)
    if(ans[i] == '#'){
      ans[i] = 'A';
      break;
    }
 
}
void B(){
  for(int i = ans.length()-1; i >= 0; i--){
    if(ans[i] == '#' && !isA(i+1) && !isA(i-1)){
      ans[i] = 'B';
      return;
    }
  }

  for(int i = 0; i < ans.length(); i++)
    if(ans[i] == '#'){
      ans[i] = 'B';
      break;
    }

}
void C(){

  if(isEmpty()){
    if(n%2) ans[(n+1)/2] = 'C';
    else ans[n/2 + 1] = 'C';
    return;
  }

  for(int i = 0; i < ans.length(); i++)
    if(ans[i] != '#'){
      if(isVacant(i+1)){
	ans[i+1] = 'C';
	return;
      }
      if(isVacant(i-1)){
	ans[i-1] = 'C';
	return;
      }
    }
}

void D(){

  int d = -1,p = -1;

  if(isEmpty()) ans[0] = 'D';
  else{
    for(int i = 0; i < ans.length(); i++){
      int nowdis;
      if(ans[i] == '#') nowdis = min(left(i),right(i));

      if(nowdis > d){
	d = nowdis;
	p = i;
      }
    }
    ans[p] = 'D';
  }
  
}

void init(){
  string tmp = "";  
  for(int i = 0; i < n; i++) tmp += "#";
  ans = tmp;

}
int main(){
  
  while(cin >> n >> m && n+m){
    init();

    for(int i = 0; i < m; i++){
      char c;
      cin >> c;
      if(c == 'A') A();
      if(c == 'B') B();
      if(c == 'C') C();
      if(c == 'D') D();
      //   cout << "test  " << ans << endl;
    }

    cout << ans << endl;
  }
  return 0;
}