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 (seat[j] != 0) {
						if (j != n - 1 && seat[j + 1] == 0) {
							seat[j + 1] = 3;
							break;
						}
						if (j != 0 && seat[j - 1] == 0) {
							seat[j - 1] = 3;
							break;
						}
					}
					if (j == n - 1)
						seat[n / 2] = 3;
				}
			}
			else if (who == 'D') {
				int maxDist = 0;
				int maxIdx = 0;

				int dist = 0;

				int fromTop = 0;
				int toLast = 0;

				for (int j = 0; j < n; ++j) {
					++dist;
					if (seat[0] != 0 && 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] = 4;
				else if (seat[0] == 0 && fromTop > maxDist && fromTop > toLast)
					seat[0] = 4;
				else
					seat[maxIdx + maxDist] = 4;
			}
		}

		for (int i = 0; i < n; ++i)
			std::cout << output[seat[i]];
		std::cout << std::endl;
		seat.clear();
	}

	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++){
			char S[10009];
			scanf("%s",&S);
			if(S[0]=='A'){
				
				for(int j=0;j<N;j++){
					if(seats[j]==0){
						seats[j]=1;
						break;
					}
				}
			}
			else if(S[0]=='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[0]=='C'){
				bool F=true;
				for(int j=0;j<N;j++){
					if(seats[j]!=0){
						if(j!=N-1&&seats[j+1]==0){
							seats[j+1]=3;F=false;
						}
						else if(j!=0&&seats[j-1]==0){
							seats[j-1]=3;F=false;
						}
					}
					if(!F)break;
				}
				if(F)seats[N/2]=3;
			}
			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 <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)
int main(){
	int n,m;
	while(cin>>n>>m,n){
		char input,chair[100];
		rep(i,n)chair[i]='#';
		rep(i,m){
			bool stop=false;
			cin>>input;
			if(input=='A'){
				rep(j,n)if(chair[j]=='#'){
					chair[j]='A';
					break;
				}
			}else if(input=='B'){
				rep(j,n)if(chair[j]=='#'&&(j==0||chair[j-1]!='A')&&(j==n-1||chair[j+1]=='A')){
					chair[j]='B';
					stop=true;
					break;
				}
				if(!stop)rep(j,n)if(chair[j]=='#'){
					chair[j]='B';
					break;
				}
			}else if(input=='C'){
				if(i==0)chair[n/2+1]='C';
				else rep(j,n)if(chair[j]=='#'&&(j==0||chair[j-1]!='#')&&(j==n-1||chair[j+1]!='#')){
					chair[j]='C';
					stop=true;
					break;
				}
				if(!stop)rep(j,n)if(chair[j]=='#'){
					chair[j]='C';
					break;
				}
			}else{
				rep(j,n)if(chair[j]=='#'&&(j==0||chair[j-1]=='#')&&(j==n-1||chair[j+1]=='#')){
					chair[j]='D';
					stop=true;
					break;
				}
				if(!stop)rep(j,n)if(chair[j]=='#'){
					chair[j]='D';
					break;
				}
			}
		}
		rep(i,n)cout<<chair[i];
		cout<<endl;
	}
}