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 <algorithm>
#define MAX 200

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 found = -1;

  for(int i = 0 ; i < n ; i++){
    if(chair[i] == 'A'){
      found = i;
    }
  }

  if(found == -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] == '#'){	
	if(i && chair[i-1] == 'A') continue;
	if(i != n-1 && chair[i+1] == 'A') continue;
	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 ; i++){
      if(chair[i] != '#'){
	if(i != n-1 && chair[i+1] == '#'){
	  chair[i+1] = 'C';
	  return;
	}
	else{
	  if(i && 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);
    for(int j = 0 ; j < n ; j++){
      if(chair[j] != '#'){
	dist = min(dist,abs(i-j));
      }
    }
    if(Breadth < dist){
      Breadth = dist;
      pos = i;
    } 
  }
  if(chair[pos] == '#'){
    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){
    chair[n/2] = 'C';
    isZero = false;
    return;
  }
  
  for(int i = 0 ; i < n ; i++){
    if(!isCanSit(i)){
      if(isCanSit(i+1)){
	if(i+1 != n){
	  chair[i+1] = 'C';
	  isZero = false;
	  return;
	}
	else{
	  chair[i-1] = 'C';
	  isZero = false;
	  return;
	}
      }
      else if(isCanSit(i-1)){
	chair[i-1] = 'C';
	isZero = false;
	return;
      }
    }
  }
}

void D(){
  if(isZero){
    chair[0] = 'D';
    isZero = false;
    return;
  }
  
  int sit;
  int dis[MAX];
  fill(dis, dis+MAX, MAX);
  for(int i = 0 ; i < n ; i++){
    if(chair[i] != '#'){
      for(int j = 0 ; j < n ; j++){
	dis[j] = min(dis[j], abs(i-j));
      }
    }
  }  
  int d = 0;
  for(int i = 0 ; i < n ; i++){
    if(dis[i] > d){
      sit = i;
      d = dis[i];
    }
  }
  chair[sit] = 'D';
  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();
    }
    display();
  }
  return 0;
}

C++: #include<bits/stdc++.h>
#define for1(i, n) for(int i = 1; i <= (n);i++)
using namespace std;
int n, m;
signed main() {
	while (cin >> n >> m, n) {
		string ans; for1(i, n + 2)ans += '#';
		for1(v1lah, m) {
			char ch; cin >> ch;
			if (ch == 'A') {
				for1(i, n)if (ans[i] == '#') { ans[i] = ch; break; }
			}
			else if (ch == 'B') {
				for (int i = n; i > 0; i--)if (ans[i] == '#' && ans[i - 1] != 'A' && ans[i + 1] != 'A') { ans[i] = ch; goto heavenb; }
				for1(i, n)if (ans[i] == '#') { ans[i] = ch; break; }
			heavenb:;
			}
			else if (ch == 'C') {
				if (v1lah == 1)ans[n / 2 + 1] = ch;
				else for1(i, n)if (ans[i] != '#') { if (i != n && ans[i + 1] == '#') { ans[i + 1] = ch; break; }if (i != 1 && ans[i - 1] == '#') { ans[i - 1] = ch; break; } }
			}
			else {
				if (v1lah == 1)ans[1] = ch;
				else {
					int a[110]; a[0] = n + 1;
					for1(i, n) {
						if (ans[i] == '#')a[i] = a[i - 1] + 1;
						else {
							a[i] = 0; int p = i - 1;
							while (p >= 1) { if (a[p] > a[p + 1] + 1)a[p] = a[p + 1] + 1; else break; p--; }
						}
					}
					int p = 1;
					for1(i, n)if (a[p] < a[i])p = i;
					ans[p] = ch;
				}
			}
		}
		cout << ans.substr(1, n) << endl;
	}
}