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
			}
		}
		for i := 0; i < n; i++ {
			if i == n-1 {
				fmt.Println(chair[i])
			} else {
				fmt.Printf("%s ", chair[i])
			}
		}
	}
}

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<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<cassert>
#include<iostream>
#include<sstream>
#include<string>
#include<vector>
#include<queue>
#include<set>
#include<map>
#include<utility>
#include<numeric>
#include<algorithm>
#include<bitset>
#include<complex>

using namespace std;

typedef long long Int;
typedef vector<int> vint;
typedef pair<int,int> pint;
#define mp make_pair

template<class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cout << *i << " "; cout << endl; }
template<class T> void chmin(T &t, T f) { if (t > f) t = f; }
template<class T> void chmax(T &t, T f) { if (t < f) t = f; }
int in() { int x; scanf("%d", &x); return x; }

int chairs[110];
int n,m;

void fa(){
	int i;
	for(i=0;i<n;i++){
		if(chairs[i]==0){
			chairs[i]=1;
			return;
		}
	}
}

void fb(){
	int i;
	for(i=n-1;i>=0;i++){
		if(chairs[i]==0&&(i==n-1||chairs[i+1]!=1)&&(i==0||chairs[i-1]!=1)){
			chairs[i]=2;
			return;
		}
	}
	for(i=0;i<n;i++){
		if(chairs[i]==0){
			chairs[i]=2;
			return;
		}
	}
}

void fc(){
	int i;
	for(i=0;i<n;i++){
		if(chairs[i]==0)continue;
		if(i!=n-1&&chairs[i+1]==0){
			chairs[i+1]=3;
			return;
		}
		if(i!=0&&chairs[i-1]==0){
			chairs[i-1]=3;
			return;
		}
	}
	if(n%2==0){
		chairs[n/2+1]=3;
		return;
	}else{
		chairs[(n+1)/2]=3;
		return;
	}
}

void fd(){
	int i,j;
	int c=0,d=0,dd;
	for(i=0;i<n;i++){
		if(chairs[i]==0){
			for(j=1;i-j>=0;j++){
				if(chairs[i-j]!=0)break;
			}
			if(i-j<0)j=n;
			dd=j;
			for(j=1;i+j<n;j++){
				if(chairs[i+j]!=0)break;
			}
			if(i+j==n)j=n;
			chmin(dd,j);
			if(d<dd){
				c=i;
				d=dd;
			}
		}
	}
	chairs[c]=4;
	return;
}

int main() {
	while(cin>>n>>m){
		int i;
		string str;
		for(i=0;i<n;i++)chairs[i]=0;
		for(i=0;i<m;i++){
			cin>>str;
			if(str[0]=='A')fa();
			if(str[0]=='B')fb();
			if(str[0]=='C')fc();
			if(str[0]=='D')fd();
		}
		for(i=0;i<n;i++){
			if(chairs[i]==0)cout<<"#";
			else if(chairs[i]==1)cout<<"A";
			else if(chairs[i]==2)cout<<"B";
			else if(chairs[i]==3)cout<<"C";
			else if(chairs[i]==4)cout<<"D";
		}
		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 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;
    } 
  }

  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 <vector>
#include <algorithm>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
vector<int> data;
int n,N;

int getLeft(){
	rep(i,n)if(data[i] == 0)return i;
	rep(i,n)cout << data[i] << " ";cout << endl;
	return -1;
}

int B(){
	for(int i=n-1;i>=0;i--){
		if(data[i] == 0 && data[i+1] != 1 && data[i-1] != 1)return i;
	}
	return getLeft();
}
int C(){
	bool f = true;
	rep(i,N)f &=(data[i+1]==0);
	if(f)return (N&1?n/2:N/2+1);
	
	rep(i,n){
		if(data[i] == 0 && data[i+1] != -1 && data[i+1])return i;
		if(data[i] == 0 && data[i-1] != -1 && data[i-1])return i;
	}
}

int most(int pos,int a){
	int i = 0;
	while(!data[pos+i*a]){
		i++;
		if(data[pos+i*a] == -1)return 999999;
	}
	return i;
}
int D(){
	int good , val = -1;
	rep(i,n){
		if(data[i] == 0){
			int t = min( most(i,-1) , most(i,1) );
			if(t > val){
				val = t;
				good = i;
			}
		}
	}
	return good;
}
int main(){
	string tab = "#ABCD";
	int m;
	while(cin >> n >> m , n ){
		N = n ; n++;
		data.resize(N+2);
		fill(data.begin(),data.end(),0);
		data[0] = -1 , data[n] = -1;
		
		rep(i,m){
			char c; cin >> c;
			switch(c){
			case 'A':
				data[getLeft()] = 1;
				break;
			case 'B':
				data[B()] = 2;
				break;
			case 'C':
				data[C()] = 3;
				break;
			case 'D':
				data[D()] = 4;
				break;
			}
		}
		rep(i,N){
			int t = data[i+1];
			cout << tab[t];
		}
		cout << endl;
	}
}