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 <cstdio>
#include <cstring>
#include <algorithm>
#include <map>
#include <vector>
using namespace std;

char seat[110];
bool is_seat = false;
int n, m;

void A(){
  for(int i = 0 ; i < n ; i++){
    if(seat[i] == '#'){
      seat[i] = 'A';
      is_seat = true;
      return;
    }
  }
}

void B(){
  bool flag = false;
  for(int i = n-1 ; i >= 0 ; i--){
    if(seat[i-1] != 'A' && seat[i+1] != 'A' && seat[i] == '#'){
      seat[i] = 'B';
      flag = true;
      is_seat = true;
      return;
    }
  }
  if(!flag){
    for(int i = 0 ; i < n ; i++){
      if(seat[i] == '#'){
	seat[i] = 'B';
	is_seat = true;
	return;
      }
    }
  }
}

void C(){
  if(!is_seat){
    if(n % 2 == 1) seat[(n+1)/2-1] = 'C';
    else seat[n/2+1-1] = 'C';
    is_seat = true;
    return;
  }
  else{
    for(int i = 0 ; i < n ; i++){
      if(seat[i] != '#'){
	if(seat[i+1] == '#'){
	  seat[i+1] = 'C';
	  is_seat = true;
	  return;
	}
	else{
	  if(seat[i-1] == '#'){
	    seat[i-1] = 'C';
	    is_seat = true;
	    return;
	  }
	}
      }
    }
  }
}

int find_near(int x){
  int cnt = 0;
  int len;
  for(int i = x-1 ;  ; i--){
    cnt++;
    if(i < 0){
      cnt = 110;
      break;
    }
    if(seat[i] != '#') break;
  }
  len = cnt;
  cnt = 0;
  for(int i = x+1 ; ; i++){
    cnt++;
    if(i >= n){
      cnt = 110;
      break;
    }
    if(seat[i] != '#') break;
  }
  len = min(len, cnt);
  return len;
}
  
  
void D(){
  bool Must_left = false;
  bool Must_next = true;
  vector<pair<int, int> > check;
  if(!is_seat){
    is_seat = true;
    seat[0] = 'D';
    return;
  }
  
  for(int i = 0 ; i < n ; i++){
    if(seat[i-1] == '#' && seat[i+1] == '#') Must_next = false;
  }
  for(int i = 0 ; i < n ; i++){
    if(seat[i] == '#'){
      int len = find_near(i);
      check.push_back(make_pair(len, i));
    }
  }
  sort(check.begin(), check.end());
  /*for(int i = 0 ; i < check.size() ; i++){
    cout << check[i].first << endl;
  }*/
  int Max = check[check.size()-1].first;
  if(check[check.size()-2].first == Max) Must_left = true; //same
  
  /*for(int i = 0 ; i < check.size() ; i++){
    if(check[i].first == check[i+1].first) Must_left = true;
  }
*/
  if(Must_left || Must_next){
    for(int i = 0 ; i < n ; i++){
      if(seat[i] == '#'){
	seat[i] = 'D';
	is_seat = true;
	return;
      }
    }
  }
  else{
    seat[check[check.size()-1].second] = 'D';
    is_seat = true;
    return;
  }
}

int main(){
  while(cin >> n >> m){
    if(n == 0 && m == 0) break;
    fill(seat, seat+110, '#');
    char p;
    int a;
    is_seat = false;
    while(m--){
      cin >> p;
      if(p == 'A') A();
      else if(p == 'B') B();
      else if(p == 'C') C();
      else if(p == 'D') D();
      /*else if(p == 'F'){
	cin >> a;
	seat[a] = 'N';
      }
      for(int i = 0 ; i < n ; i++)cout << seat[i];
      cout << endl;*/
    }
    for(int i = 0 ; i < n ; i++)cout << seat[i];
    cout << endl;
  }
  return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <complex>
#include <cstring>
#include <cstdlib>
#include <string>
#include <cmath>
#include <queue>
using namespace std;

#define REP(i,n) for(int i=0;i<(int)n;++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
const int INF = 1<<29;

int main() {
  int n, m;
  while(cin>>n>>m,n+m) {
    vector<char> v(n, '#');
    REP(j,m) {
      char c;
      cin >> c;
      if (c=='A') {
        REP(i,n) {
          if (v[i] == '#') {
            v[i] = c;
            break;
          }
        }
      } else if (c=='B') {
        bool f = 0;
        for(int i=n-1;i>=0; --i) {
          if (v[i] == '#') {
            if (i<n-1&&v[i+1]=='A' || i>0&&v[i-1]=='A')
              continue;
            v[i] = c;
            f = 1;
            break;
          }
        }
        if (!f) {
          REP(i,n) {
            if (v[i] == '#') {
              v[i] = c;
              break;
            }
          }
        }
      } else if (c=='C') {
        bool f = 0;
        REP(i,n) {
          if (v[i] != '#') {
            if (i<n-1 && v[i+1]=='#') {
              v[i+1] = c;
              f = 1;
              break;
            }
            
            if (i>0&&v[i-1]=='#') {
              v[i-1] = c;
              f = 1;
              break;
            }
          }
        }
        if (!f) {
          if (n%2)
            v[(n+1)/2-1] = c;
          else
            v[n/2] = c;
        }
      } else {
        vector<int> dst(n,INF);
        REP(i,n) REP(j,n)
          if (v[j] != '#')
            dst[i] = min(dst[i], abs(i-j));
        int ma = 0;
        int p = 0;
        REP(i,n) {
          if (v[i] == '#' &&ma < dst[i]) {
            ma = dst[i];
            p = i;
          }
        }
        v[p] = c;
      }
       // REP(i,n)
       //   cout << v[i];
       // cout << endl;

    }
    REP(i,n)
      cout << v[i];
    cout << endl;
  }
}