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 <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(i == n-1){
	if(chair[i-1] != 'A'){
	  chair[i] = 'B';
	  return;
	}
      }else{
	if(chair[i] == '#' && chair[i-1] != 'A' && 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';
    }
  }
  else{
    for(int i = 0 ; i < n ; i++){
      if(chair[i] != '#' && i != n-1 && 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;
  int cnt = count(chair,chair+n,'#');

  if(cnt == n){
    chair[0] = 'D';
    return;
  } 

  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;
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <map>
using namespace std;

#define MAX 110
typedef pair<int, int> P;
int n, m;
char chair[MAX];
bool isZero;

bool isCanSit(int x){
  if(chair[x] == '#') return true;
  return false;
}

void A(){
  for(int i = 0 ; i < n ; i++){
    if(isCanSit(i)){
      chair[i] = 'A';
      isZero = false;
      return;
    }
  }
}

bool isNext(int x){
  if(chair[x-1] == 'A' || chair[x+1] == 'A') return true;
  return false;
}

void B(){
  for(int i = n-1 ; i >= 0 ; i--){
    if(isCanSit(i) && !isNext(i)){
      chair[i] = 'B';
      isZero = false;
      return;
    }
  }
  
  for(int i = 0 ; i < n ; i++){
    if(isCanSit(i)){
      chair[i] = 'B';
      isZero = false;
      return;
    }
  }
}

void C(){
  if(isZero){
    if(n % 2 == 1) chair[(n+1)/2] = 'C';
    else chair[n/2+1] = 'C';
    isZero = false;
    return;
  }
  
  for(int i = 0 ; i < n ; i++){
    if(!isCanSit(i)){
      if(isCanSit(i+1)){
	chair[i+1] = 'C';
	isZero = false;
	return;
      }
      else if(isCanSit(i-1)){
	chair[i-1] = 'C';
	isZero = false;
	return;
      }
    }
  }
}

bool DcanSitNoneNext(int x){
  if(x == n-1){
    if(chair[x-1] == '#') return true;
  }
  if(x == 0){
    if(chair[x+1] == '#') return true;
 }  
  if(chair[x-1] == '#' && chair[x+1] == '#') return true;
  return false;
}

int calcDisD(int x){
  int ret, cnt = 0;
  for(int i = x-1 ; i >= 0 ; i--){ // left
    if(chair[i] != '#') break;
    else cnt++;
  }
  
  ret = cnt;
  cnt = 0;
  
  for(int i = x+1 ; i < n ; i++){ // right
    if(chair[i] != '#') break;
    else cnt++;
  }
  
  ret = max(ret, cnt);
  return ret;
}

void D(){
  if(isZero){
    chair[0] = 'D';
    isZero = false;
    return;
  }

  bool Cansit = false;
  for(int i = 0 ; i < n ; i++){
    if(isCanSit(i)){
      if(DcanSitNoneNext(i)) Cansit = true; // 一つでも座れる場所がある
    }
  }

  if(!Cansit){
    for(int i = 0 ; i < n ; i++){
      if(isCanSit(i)){
	chair[i] = 'D';
	isZero = false;
	return;
      }
    }
  }
  
  vector<P> dis;
  
  for(int i = 0 ; i < n ; i++){
    if(isCanSit(i)){
      dis.push_back(P(-calcDisD(i), i));
    } 
  }
  
  sort(dis.begin(), dis.end());
  chair[dis[0].second] = 'D';
  isZero = false;
  return;
}

void init(){
  fill(chair, chair+MAX, '#');
  isZero = true;
}

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

int main(){
  char p;
  while(cin >> n >> m){
    if(n == 0 && m == 0) break;
    init();
    for(int i = 0 ; i < m ; i++){
      cin >> p;
      if(p == 'A') A();
      if(p == 'B') B();
      if(p == 'C') C();
      if(p == 'D') D();
    }
    display();
  }
  return 0;
}