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<iostream>

char *chr, *peo;
int n,m;

void sitA(int i){
	for(int k=0; k<n; k++){
		if(chr[k] == '#'){
			chr[k] = 'A';
				break;
		}
	}
	return;
}

void sitB(int i){
	if(chr[n-1]=='#' && chr[n-2] != 'A'){
		chr[n-1] = 'B';
		return;
	}
	for(int k=n-2; k>0; k--){
		if(chr[k] == '#' && chr[k-1] != 'A' && chr[k+1] != 'A'){
			chr[k] = 'B';
			return;
		}
	}
	for(int k=0; k<n; k++){
		if(chr[k] == '#'){
			chr[k] = 'B';
			return;
		}
	}
}

void sitC(int i){
	if(i==0){
		if(n%2 == 0)
			chr[n/2] = 'C';
		else
			chr[(n-1)/2] = 'C';
		return;
	}
	for(int k=0; k<n; k++){
		if(chr[k] == '#')
			continue;
		if(k<n-1){
			if(chr[k+1] == '#'){
				chr[k+1] = 'C';
				return;
			}
		}
		if(k>0){
			if(chr[k-1] == '#'){
				chr[k-1] = 'C';
				return;
			}
		}
	}
}

void sitD(int i){
	if(i==0){
		chr[0] = 'D';
		return;
	}
	int ans,tmp=0,tmp1,tmp2;
	for(int k=0; k<n; k++){
		if(chr[k] == '#'){
			ans = k;
			break;
		}
	}
	for(int k=ans; k<n; k++){
		if(chr[k] != '#')
			continue;
		tmp1=0;
		tmp2=0;
		if(k<n-1){
			for(int t=k+1; t<n; t++){
				if(chr[t] != '#')
					break;
				tmp1++;
				if(t == n-1)
					tmp1 = n+1;
			}
		}
		else
			tmp1 = n+1;
		if(k>0){
			for(int t=k-1; t>=0; t--){
				if(chr[t] != '#')
					break;
				tmp2++;
				if(t == 0)
					tmp2 = n+1;
			}
		}
		else
			tmp2 = n+1;
		if(tmp1 > tmp2)
			tmp1 = tmp2;
		if(tmp1 > tmp){
			tmp = tmp1;
			ans = k;
		}
	}
	chr[ans] = 'D';
}

void solve(){
	char tmp;
	for(int i=0; i<m; i++){
		tmp = peo[i];
		switch(tmp){
		case 'a':
		case 'A':
			sitA(i);
			break;
		case 'b':
		case 'B':
			sitB(i);
			break;
		case 'c':
		case 'C':
			sitC(i);
			break;
		case 'd':
		case 'D':
			sitD(i);
			break;
		}
	}
	std::cout << chr << std::endl;
}

int main(){
	while(true){
		std::cin >> n >> m;
		if(!n && !m)
			return 0;
		chr = new char[n];
		peo = new char[m];
		for(int i=0; i<n; i++){
			chr[i] = '#';
			if(i>=m)
				chr[i] = '#';
			else
				std::cin >> peo[i];
		}
		solve();
		delete[] chr;
		delete[] peo;
	}
}

C++: #include<bits/stdc++.h>
#define INF (1e9)
#define N 102
using namespace std;
int n,m;
char a,ans[N];
 
void UpdateA(){
  for(int i=1;i<=n;i++)
    if(ans[i]=='#'){
      ans[i]='A';
      return;
    }
}
 
void UpdateB(){
  for(int i=n;i>0;i--){
    if(ans[i]!='#')continue;
    if(ans[i-1]!='A'&&ans[i+1]!='A'){
      ans[i]='B';
      return;
    }
  }
  for(int i=1;i<=n;i++)
    if(ans[i]=='#'){
      ans[i]='B';
      return;
    }
}
 
void UpdateC(){
  for(int i=1;i<=n;i++){
    if(ans[i]=='#')continue;
    if(ans[i+1]=='#'&&i+1<=n){
      ans[i+1]='C';
      return;
    }
    if(ans[i-1]=='#'&&1<=i-1){
      ans[i-1]='C';
      return;
    }
  }
  if(n%2)ans[(n+1)/2]='C';
  else ans[n/2+1]='C';
}
 
void UpdateD(){
  int d[2][N],cnt=INF,maxd;
  for(int i=0;i<2;i++)
    for(int j=1;j<=n;j++)d[i][j]=INF;
  for(int i=1;i<=n;i++){
    if(ans[i]!='#')cnt=0;
    else cnt++;
    d[0][i]=cnt;
  }
  cnt=INF;
  for(int i=n;i>=1;i--){
    if(ans[i]!='#')cnt=0;
    else cnt++;
    d[1][i]=min(cnt,d[0][i]);
  }
  maxd=0;
  for(int i=1;i<=n;i++)maxd=max(maxd,d[1][i]);
  if(maxd>=INF){
    ans[1]='D';
    return; 
  }else{
    for(int i=1;i<=n;i++){
      if(d[1][i]==maxd){
    ans[i]='D';
    return;
      }
    }
  }
}
 
int main(){
  while(1){
    cin>>n>>m;
    if(!n&&!m)break;
    for(int i=1;i<=n;i++)ans[i]='#';
    for(int i=0;i<m;i++){
      cin>>a;
      if(a=='A')UpdateA();
      if(a=='B')UpdateB();
      if(a=='C')UpdateC();
      if(a=='D')UpdateD();
    }
    for(int i=1;i<=n;i++)cout<<ans[i];
    cout<<endl;  
  }
  return 0;
}

C++: #include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

int chair[100],pos,n,m;

int d(char c){
	if(c == 'A'){
		for(int i=0;i<n;i++){
			if(chair[i] == -1) return i;
		}
	}else if(c == 'B'){
		for(int i=n-1;i>=0;i--){
			if(chair[i] == -1){
				if(i == 0){
					if(chair[i+1] != 'A') return i;
				}else if(i == n-1){
					if(chair[i-1] != 'A') return i;
				}else{
					if(chair[i-1] != 'A' && chair[i+1] != 'A') return i;
				}
			}
		}
		for(int i=0;i<n;i++){
			if(chair[i] == -1) return i;
		}
	}else if(c == 'C'){
		for(int i=0;i<n;i++){
			if(chair[i] != -1){
				if(i+1 < n && chair[i+1] == -1) return i+1;
				if(0 <= i-1 && chair[i-1] == -1) return i-1;
			}
		}
		return n/2+1;
	}else{
		int d_min_max = 0,d_pos=-1,tmp;
		for(int i=0;i<n;i++){
			//ó¢Ä¢éÖqÆÅàß¢lÌ£ðßé
			
			if(chair[i] == -1){
				int dist_min = 1 << 28;
				if(i != n-1){
					tmp = 1;
					while(i+tmp < n && chair[i+tmp] == -1) tmp++;
					if(i+tmp<n){
						dist_min = min(dist_min, tmp);
					}
				}
				if(i != 0){
					tmp = 1;
					while(i-tmp >= 0 && chair[i-tmp] == -1) tmp++;
					if(i-tmp >= 0){
						dist_min = min(dist_min, tmp);
					}
				}
				if(d_min_max < dist_min){
					d_min_max = dist_min;
					d_pos = i;
				}
			}
		}
		if(d_pos != -1) return d_pos;
		for(int i=0;i<n;i++){
			if(chair[i] == -1) return i;
		}
	}
}

int main(){
	while(cin >> n >> m){
		if(!(n|m)) break;
		char c;
		memset(chair, -1, sizeof(chair));
		for(int i=0;i<m;i++){
			cin >> c;
			chair[d(c)] = c;
		}
		for(int i=0;i<n;i++){
			if(chair[i] == -1){
				cout << "#";
			}else{
				cout << (char)chair[i];
			}
		}
		cout << endl;
	}
}