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(){
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]!='#'||0<i&&a[i-1]=='A'||i<n-1&&a[i+1]=='A')continue;
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++: // 2011/07/21 Tazoe

#include <iostream>
using namespace std;

int main()
{
	while(true){
		int n, m;
		cin >> n >> m;
		if(n==0&&m==0)
			break;

		char C[100];
		for(int i=0; i<n; i++)
			C[i] = '#';

		for(int i=0; i<m; i++){
			char a;
			cin >> a;

			bool flg;
			switch(a){
				case 'A':
					for(int j=0; j<n; j++)
						if(C[j]=='#'){
							C[j] = a;
							break;
						}
					break;
				case 'B':
					flg = false;
					for(int j=n-1; j>=0; j--)
						if(C[j]=='#'&&(j==0||C[j-1]!='A')&&(j==n-1||C[j+1]!='A')){
							C[j] = a;
							flg = true;
							break;
						}
					if(!flg)
						for(int j=0; j<n; j++)
							if(C[j]=='#'){
								C[j] = a;
								break;
							}
					break;
				case 'C':
					if(i==0){
						C[n/2] = a;
						break;
					}
					for(int j=0; j<n; j++)
						if(C[j]!='#'&&j!=n-1&&C[j+1]=='#'){
							C[j+1] = a;
							break;
						}
						else if(C[j]!='#'&&j!=0&&C[j-1]=='#'){
							C[j-1] = a;
							break;
						}
					break;
				case 'D':
					if(i==0){
						C[0] = a;
						break;
					}
					int D[100];
					for(int j=0; j<n; j++){
						if(C[j]!='#'){
							D[j] = 0;
							continue;
						}
						int d = 0;
						for(int k=j; k<n; k++, d++)
							if(C[k]!='#')
								break;
						D[j] = d;
						d = 0;
						for(int k=j; k>=0; k--, d++)
							if(C[k]!='#')
								break;
						if(d<D[j])
							D[j] = d;
					}
					int maxD = -1;
					int maxI = -1;
					for(int j=0; j<n; j++)
						if(maxD<D[j]){
							maxD = D[j];
							maxI = j;
						}
					C[maxI] = a;				
					break;
			}
		}

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

	return 0;
}

C++: #include <iostream>
#include <algorithm>

#define MAX 100

using namespace std;

int n,m;
char chair[MAX+1];
 
void A(){
  for(int i = 0 ; i < n ; i++){
    if(chair[i] == '#'){
      chair[i] = 'A';
      return;
    }
  }
}

void B(){
  int pos = -1;
  
  for(int i = 0 ; i < n ; i++){
    if(chair[i] == 'A'){
      pos = i;
      break;
    }
  }
  
  if(pos == -1){
    for(int i = n-1 ; i >= 0 ; i--){
      if(chair[i] == '#'){
        chair[i] = 'B';
        return;
      }
 
    }
 
  }
  else{
    for(int i = n-1 ; i > 0 ; i--){
      if(chair[i] == '#' && chair[i-1] != 'A'){
        chair[i] = 'B';
        return;
      }
    }
 
    for(int i = 0 ; i < n ; i++){
      if(chair[i] == '#'){
        chair[i] = 'B';
        return;
      }
    }
  }
}
 
void C(){
  int cnt = count(chair,chair+n,'#');
 
  if(cnt == n){
    if(n % 2){
      chair[(n+1)/2] = 'C';
    }
    else{
      chair[n/2+1] = 'C';
    }
    return;
  }
  else{
    for(int i = 0 ; i < n-1 ; i++){
      if(chair[i] != '#' && chair[i+1] == '#'){
        chair[i+1] = 'C';
        return;
      }else if(chair[i] != '#' && i > 0 && chair[i-1] == '#'){
        chair[i-1] = 'C';
        return;
      }
    }
  }
}
 
void D(){
  int Breadth = 0, pos;
 
  for(int i = 0 ; i < n ; i++){
    int dist = (1<<29);
    if(chair[i] != '#') continue;
    for(int j = 0 ; j < n ; j++){
      if(chair[j] != '#'){
        dist = min(dist,abs(j-i));
      }
    }
    if(Breadth < dist){
      Breadth = dist;
      pos = i;
    } 
  }
 
  chair[pos] = 'D';
}
 
int main(){
  while(cin >> n >> m , n | m){
    char person;
 
    for(int i = 0 ; i < n ; i++)
      chair[i] = '#';
 
    for(int i = 0 ; i < m ; i++){
      cin >> person;
      
      switch(person){
      case 'A': 
        A(); 
        break;
      case 'B': 
        B(); 
        break;
      case 'C': 
        C(); 
        break;
      case 'D': 
        D(); 
        break;
      }
 
    }
 
    for(int i = 0 ; i < n ; i++){
      cout << chair[i];
    }
    cout << endl;
  }
 
  return 0;
}