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 <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 pos = -1;

  for(int i = 0 ; i < n ; i++){
    if(chair[i] == 'A'){
      pos = i;
    }
  }

  if(pos == -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] == '#' && chair[i-1] != 'A'){
	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-1 ; i++){

      if(chair[i] != '#'){

	if(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<bits/stdc++.h>
using namespace std;
int main()
{
	int N,M;
	while(true){
		scanf("%d%d",&N,&M);
		if(N==0&&M==0)return 0;
		int seats[10009];
		for(int i=0;i<10009;i++){
			seats[i]=0;
		}
		
		for(int i=0;i<M;i++){
			string S;
			cin >> S;
			if(S=="A"){
				for(int j=0;j<N;j++){
					if(seats[j]==0){
						seats[j]=1;
						break;
					}
				}
			}
			else if(S=="B"){
				bool F=true;
				for(int j=N-1;j>=0;j--){
					if(F&&(j==0||seats[j-1]!=1)&&(j==N-1||seats[j+1]!=1)&&seats[j]==0){
						seats[j]=2;F=false;
					}
				}
				if(F){
					for(int j=0;j<N;j++){
						if(seats[j]==0){
							seats[j]=2;
							break;
						}
					}
				}
			}
			else if(S=="C"){
				bool F=true;bool FF=true;
				for(int j=0;j<N;j++){
					if(seats[j]!=0){
						FF=false;
					}
				}
				if(FF){
					seats[N/2]=3;
					F=false;
				}
				if(!F)continue;
				for(int j=0;j<N;j++){
					if(seats[j]!=0){
						if(F&&j!=N-1&&seats[j+1]==0){
							seats[j+1]=3;F=false;
						}
						else if(F&&j!=0&&seats[j-1]==0){
							seats[j-1]=3;F=false;
						}
					}
				}
			}
			else{
				int maxd=-1,maxi;
				bool F=true;
				for(int i=0;i<N;i++){
					if(seats[i]!=0)F=false;
				}
				if(F){
					seats[0]=4;
					continue;
				}
				for(int i=0;i<N;i++){
					int d=0;
					if(seats[i]!=0)d=-114;
					int l=i;int r=i;
					while((l<0||seats[l]==0)&&(r>=N||seats[r]==0)){
						d++;l--;r++;
					}
					if(maxd<d){
						maxd=d;
						maxi=i;
					}
				}
				seats[maxi]=4;
			}
			
		}
		for(int i=0;i<N;i++){
				if(seats[i]==0)printf("#");
				if(seats[i]==1)printf("A");
				if(seats[i]==2)printf("B");
				if(seats[i]==3)printf("C");
				if(seats[i]==4)printf("D");
			}
			printf("\n");
	}
}

C++: #include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <cmath>
#include <stack>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <list>
#include <iostream>
#include <sstream>
#include <climits>
#include <cfloat>
#include <complex>

using namespace std;

int main()
{
	int n, m;
	while (scanf("%d %d", &n, &m), n | m)
	{
		char t[102];	// 1~n
		memset(t, '#', sizeof(t));
		for (int w = 0; w < m; ++w)
		{
			char c;
			scanf(" %c", &c);
			if (c == 'A')
			{
				int i;
				for (i = 1; t[i] != '#'; ++i)
					;
				t[i] = 'A';
			}
			else if (c == 'B')
			{
				int i;
				for (i = n; i >= 1; --i)
					if (t[i-1] != 'A' && t[i] == '#' && t[i+1] != 'A')
						break;
				if (i == 0)
					for (i = 1; t[i] != '#'; ++i)
						;
				t[i] = 'B';
			}
			else if (c == 'C')
			{
				int i;
				if (w == 0)
					i = n / 2 + 1;
				else
				{
					for (i = 1; ; ++i)
						if (t[i] != '#')
							if (t[i+1] == '#')
							{
								++i;
								break;
							}
							else if (t[i-1] == '#' && i > 1)
							{
								--i;
								break;
							}
				}
				t[i] = 'C';
			}
			else
			{
				int i;
				if (w == 0)
					i = 1;
				else
				{
					int dis[102];
					for (int j = 0; j < 102; ++j)
						dis[j] = 100000000;
					for (int j = 1; j <= n; ++j)
					{
						if (t[j] != '#')
							for (int k = 1; k <= n; ++k)
								dis[k] = min(dis[k], abs(j-k));
					}
					int d = 0;
					for (int j = 1; j <= n; ++j)
					{
						if (dis[j] > d)
						{
							i = j;
							d = dis[j];
						}
					}
				}
				t[i] = 'D';
			}
		}
		for (int i = 1; i <= n; ++i)
			putchar(t[i]);
		printf("\n");
	}

	return 0;
}