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<stdio.h>
#include<string.h>
#include<math.h>
int main(void)
{
	int a,s,d[100],f,g,h,i,j,k,c,x[3];
	char z[2];
	while(1){
		scanf("%d %d",&a,&s);
		if(a==0&&s==0) break;
		f=a;
		for(i=0;i<a;i++) d[i]=0;
		for(i=0;i<s;i++){
			scanf("%s",z);
			if(z[0]=='A'){
				for(j=0;j<a;j++){
					if(d[j]==0){
						d[j]=1;
						f--;
						break;
					}
				}
			}
			else if(z[0]=='B'){
				g=0;
				for(j=a-1;j>=0;j--){
					if(d[j]==0&&d[j+1]!=1&&d[j-1]!=1){
						d[j]=2;
						f--;
						break;
					}
					if(d[j]==0) g++;
					if(f==g){
						d[j]=2;
						f--;
						break;
					}
				}
			}
			else if(z[0]=='C'){
				if(a==f){
					if(a%2==1){
						d[((a+1)/2)-2]=3;
						f--;
					}
					else if(a%2==0){
						d[a/2-1]=3;
						f--;
					}
				}
				else{
					for(j=0;j<a;j++){
						if(d[j]>0){
							if(d[j+1]==0){
								d[j+1]=3;
								f--;
								break;
							}
							else if(d[j-1]==0){
								d[j-1]=3;
								f--;
								break;
							}
						}
					}
				}
			}
			else if(z[0]=='D'){
				h=0;
				c=0;
				x[0]=0;
				x[1]=0;
				for(j=a-1;j>=0;j--){
					if(d[j]>0&&h==0) h=j;
					else if(d[j]>0&&h!=0) c=j;
					if(c>0&&h>0){
						if(((h-1)-c)%2==1){
							if(x[0]<=((h-1)-c)/2){
								x[0]=((h-1)-c)/2;
								x[1]=h-((h-1)-c)/2;
							}
						}
						else if(((h-1)-c)%2==0){
							if(x[0]<=((h-1)-c)/2-1){
								x[0]=((h-1)-c)/2-1;
								x[1]=h-((h-1)-c)/2-1;
							}
						}
					}
				}
				d[x[1]]=4;
				f--;
			}
		}
		for(i=0;i<a;i++){
			if(d[i]==0) printf("#");
			else if(d[i]==1) printf("A");
			else if(d[i]==2) printf("B");
			else if(d[i]==3) printf("C");
			else if(d[i]==4) printf("D");
		}
		printf("\n");
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
#define BLANK '#'
int main()
{
  int n,m;
  int i,j,blank,maximum,d1,d2;
  bool f;
  char c;
  vector<char> seat;
  while(cin>>n>>m,n|m){
    seat.resize(n);
    for(i=0;i<n;i++)seat[i]=BLANK;
    while(m-->0){
      cin>>c;
      switch(c){
	// A
	case 'A':
	  for(i=0;i<n;i++)
	    if(seat[i]==BLANK){seat[i]=c;break;}
	  break;
	// B
	case 'B':
	  for(i=n;i-->0;){
	    if(seat[i]==BLANK){
	      f=true;
	      if(i+1<n&&seat[i+1]=='A')f=false;
	      if(i>0&&seat[i-1]=='A')f=false;
	      blank=i;
	      if(f){seat[blank]=c;break;}
	    }
	  }
	  if(seat[blank]==BLANK)seat[blank]=c;
	  break;
	// C
	case 'C':
	  f=true;
	  for(i=0;i<n;i++)if(seat[i]!=BLANK)f=false;
	  if(f){seat[n/2]=c;break;}
	  for(i=0;i<n;i++){
	    if(seat[i]!=BLANK){
	      if(i+1<n&&seat[i+1]==BLANK){seat[i+1]=c;break;}
	      if(i>0&&seat[i-1]==BLANK){seat[i-1]=c;break;}
	    }
	  }
	  break;
	// D
	case 'D':
	  maximum=-1;
	  blank=0;
	  for(i=0;i<n;i++){
	    if(seat[i]==BLANK){
	      f=false;
	      for(d1=0,j=i;j-->0;d1++)if(seat[j]!=BLANK){f=true;break;}
	      if(!f)d1=n;
	      f=false;
	      for(d2=0,j=i;++j<n;d2++)if(seat[j]!=BLANK){f=true;break;}
	      if(!f)d2=n;
	      if(maximum<min(d1,d2)){maximum=min(d1,d2);blank=i;}
	    }
	  }
	  seat[blank]=c;
	  break;
      }
    }
    for(i=0;i<n;i++)cout<<seat[i];
    cout<<endl;
  }
}