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[:len(chair)-1], ""))
	}
}

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>
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(0<i&&a[i-1]!='A'&&i<n-1&&a[i+1]!='A'){a[i]='B';break;}
else if(0<i&&a[i-1]!='A'){a[i]='B';break;}
else if(i<n-1&&a[i+1]!='A'){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];
for(k=0;k<n;k++)cout<<" "<<d[k];cout<<endl;
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++: #include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int n;
string seats;
char set_left(char ch) { for (char &c: seats) if (c == '#') return c = ch; }
char setB() {
    for (int i = n - 1; i >= 0; i--) {
        bool ok = seats[i] == '#';
        if (i + 1 < n) ok &= seats[i + 1] != 'A';
        if (i) ok &= seats[i - 1] != 'A';
        if (ok) return seats[i] = 'B';
    }
    return set_left('B');
}
char setC() {
    if (seats == string(n, '#')) return seats[n / 2] = 'C';
    for (int i = 0; i < n; i++) if (seats[i] != '#') {
        if (i + 1 < n && seats[i + 1] == '#') return seats[i + 1] = 'C';
        if (i && seats[i - 1] == '#') return seats[i - 1] = 'C';
    }
}
char setD() {
    if (seats == string(n, '#')) return seats[0] = 'D';
    vector<int> dist(n, n);
    for (int i = 0; i < n; i++) if (seats[i] != '#') {
        for (int j = i; j < n; j++) dist[j] = min(dist[j], j - i);
        for (int j = i; j >= 0; j--) dist[j] = min(dist[j], i - j);
    }
    int idx = max_element(dist.begin(), dist.end()) - dist.begin();
    return seats[idx] = 'D';
}

int main() {
    int m;
    while (cin >> n >> m, n) {
        seats = string(n, '#');
        while (m--) {
            char c; cin >> c;
            if (c == 'A') set_left('A');
            if (c == 'B') setB();
            if (c == 'C') setC();
            if (c == 'D') setD();
        }
        cout << seats << endl;
    }
    return 0;
}

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] == '#'){
      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;
	  }
	}
      }
    }
  }
}

void D(){
  bool Must_left = 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_left = false;
  }
  for(int i = 0 ; i < n ; i++){
    if(seat[i] == '#'){
      int cnt = 0;
      int len;
      for(int j = i-1 ; seat[j] == '#' || j >= 0; j--) cnt++;
      len = cnt; cnt = 0;
      for(int j = i+1 ; seat[j] == '#' || j < n ; j++) cnt++;
      len = max(len, cnt);
      check.push_back(make_pair(len, i));
    }
  }
  sort(check.begin(), check.end());
  for(int i = 0 ; i < check.size() ; i++){
    if(check[i].first == check[i+1].first) Must_left = true;
  }

  if(Must_left){
    for(int i = 0 ; i < n ; i++){
      if(seat[i] == '#'){
	seat[i] = 'D';
	is_seat = true;
	return;
      }
    }
  }
  else{
    seat[check[0].second] = 'D';
    is_seat = true;
    return;
  }
}

int main(){
  while(cin >> n >> m){
    if(n == 0 && m == 0) break;
    fill(seat, seat+n, '#');
    char p;
    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();
      /*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;
}