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 <vector>
#include <algorithm>

int main() {
	int n, m;
	const char output[] = { '#', 'A', 'B', 'C', 'D' };
	std::vector<int> seat;
	
	while (std::cin >> n >> m) {
		if (n == 0 && m == 0)
			break;

		seat.resize(n);

		for (int i = 0; i < m; ++i) {
			char who;
			std::cin >> who;

			if (who == 'A') {
				for (int j = 0; j < n; ++j) {
					if (seat[j] == 0) {
						seat[j] = 1;
						break;
					}
				}
			}
			else if (who == 'B') {
				int firstEmpty = 334;
				bool isRightA = false;

				for (int j = n - 1; j >= 0; --j) {
					if (seat[j] == 0) {
						firstEmpty = std::min(firstEmpty, j);
						if (!isRightA && (j == 0 || seat[j - 1] != 1)) {
							seat[j] = 2;
							break;
						}
					}
					isRightA = seat[j] == 1;
				}
			}
			else if (who == 'C') {
				for (int j = 0; j < n; ++j) {
					if (j != 0) {
						if (j != n - 1 && seat[j + 1] == 0) {
							seat[j + 1] = 2;
							break;
						}
						if (j != 0 && seat[j - 1] == 0) {
							seat[j - 1] = 2;
							break;
						}
					}
					if (j == n - 1)
						seat[n / 2] = 2;
				}
			}
			else if (who == 'D') {
				int maxDist = 334;
				int maxIdx = 0;

				int dist = 0;

				int fromTop = 0;
				int toLast = 0;

				for (int j = 0; j < n; ++j) {
					++dist;
					if (j == n - 1)
						toLast = dist;

					if (seat[j] != 0) {
						if (fromTop == 0)
							fromTop = j + 1;

						if (maxDist < dist / 2) {
							maxDist = dist / 2;
							maxIdx = j - dist;
						}
						dist = 0;
					}
				}

				if (seat[n - 1] == 0 && toLast > maxDist && fromTop < toLast)
					seat[n - 1] = 3;
				else if (seat[0] == 0 && fromTop > maxDist && fromTop > toLast)
					seat[0] = 3;
				else
					seat[maxIdx + maxDist] = 3;
			}
		}

		for (int i = 0; i < m; ++i)
			std::cout << output[i];
		std::cout << std::endl;
	}

	return 0;
}

C++: #include<iostream>
using namespace std;

int main(){

  int n,m;
  int j;
  char chair[100];
  char p;
  int left,max,m_left;

  while(1){

    cin >> n >> m;
    if(n==0 && m==0)
      break;

    for(int i=0; i<n; i++)
      chair[i]='#';


    for(int i=0; i<m; i++){

    cin >> p;

    if(p=='A'){
      j=0;
      while(chair[j]!='#')
	j++;

      chair[j]='A';
    }///////////////////////A

    else if(p=='B'){
      j=n-1;
      while(1){
	if(chair[j]=='#'){
	  if(j==n-1 && chair[j-1]!='A')
	    break;

	  else if(j>0 && chair[j+1]!='A' && chair[j-1]!='A')
	    break;

	  else if(chair[j+1]!='A')
	    break;
	  
	  else if(j==0){
	    j=-1;
	    break;
	  }
	}
	j--;
      }

      if(j!=-1)
	chair[j]='B';

      else{
	j=0;
	while(chair[j]!='#')
	  j++;

	chair[j]='B';
      }

    }////////////////////////B


    else if(p=='C'){
      j=0;
      while(1){
	if(chair[j]!='#'){
	  if(chair[j+1]=='#'&&j!=n-1){
	    j++;
	    break;
	  }
	  else if(j!=0 && chair[j-1]=='#'){
	    j--;
	    break;
	  }
	}

	if(j==n-1){
	  j=-1;
	  break;
	}

	j++;
      }

      if(j==-1){
	if(n%2==1)
	  j=(n+1)/2-1;
	else
	  j=n/2;
      }

      chair[j] = 'C';
    }///////////////////////C


    else{
      left=0; max=1; m_left=0;
      j=1;
      while(j<=n-1){
	if(chair[j]!='#'){
	  left=j;

	  j++;
	}

	if(max < j-left){
	  max = j-left;
	  m_left = left;
	}

	j++;
      }

      if(max <= 2){
	j=0;
	while(j<n && chair[j] != '#')
	  j++;
      }

      else if(max == n-1)
	j=0;

      else{
	if(max%2==1)
	  j=m_left+max/2 +1;
	else
	  j=m_left+max/2;
      }


      chair[j]='D';

    }///////////////////////D

    }


    for(int i=0; i<n; i++)
      cout << chair[i];


    cout << endl;

  }


  return 0;

}