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
			}
		}
		fmt.Println(strings.Join(chair, ""))
	}
}

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++: #define _CRT_SECURE_NO_WARNINGS
#include<cstdio>
#include<cmath>
#include<sstream>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<utility>
#include<iomanip>
#include<vector>
#include<string>
#include<queue>
#include<stack>
#include<map>
#include<list>
#include<set>
#include<stdlib.h>
#include<time.h>
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define repp(i,k,n) for(int i=k;i<(int)n;i++)
using namespace std;
const int dx[4] = { 0, 1, 0, -1 }, dy[4] = { 1, 0, -1, 0 };
const int dX[8] = { 0, 1, 1, 1, 0, -1, -1, -1 }, dY[8] = { 1, 1, 0, -1, -1, -1, 0, 1 };
struct XY
{
	int x;
	int y;
};
vector<long long> Prime;
void SieveOfEratosthenes(long long e) //setup vector<long long>Prime(~x)
{
	bool *x=new bool[e];
	for (long long i = 0; i <= e; i++)x[i] = false;
	for (long long i = 2; i <= e; i++)
	{
		if (x[i]);
		else
		{
			Prime.push_back(i);
			for (long long l = i; l <= e; l += i)x[l] = true;
		}
	}
	return;
}
int StringToInt(string a)
{
	stringstream ss;
	int x;
	ss << a;
	ss >> x;
	return x;
}
string IntToString(int a)
{
	stringstream ss;
	string x;
	ss << a;
	ss >> x;
	return x;
}
int GetRandom(int MIN, int MAX)
{
	static int flag=0;
	if (flag == 0)
	{
		srand((unsigned int)time(NULL));
		flag = 1;
	}
	return MIN + (int)(rand()*(MAX - MIN + 1.0) / (1.0 + RAND_MAX));
}
int in()
{
	int x;
	cin >> x;
	return x;
}
void outl(string a)
{
	cout << a << endl;
	return;
}
void outl(int a)
{
	cout << a << endl;
	return;
}
void out(string a)
{
	cout << a;
	return;
}
void out(int a)
{
	cout << a;
	return;
}
/*
(char)'1' - (int)48 = (int)1;
???1111181
 (char)A=(int)65
 (char)a=(int)97
 */
////////////////////////////////////////////////////

int main()
{
	int n, m;
	while (cin >> n >> m&&n&&m)
	{
		vector<char> c;
		rep(i, n)c.push_back('#');
		rep(i, m)
		{
			char now;
			cin >> now;
			bool wan = false;
			bool ok = true;
			switch (now)
			{
			case 'A':
				for (int l = 0;l<c.size(); l++)
				{
					if (c[l] == '#')
					{
						c[l] = 'A';
						break;
					}

				}
				break;
			case'B':
				for (int l = c.size() - 1; l >= 0; l--)
				{
					if (c[l] == '#'&&c[max(0, l - 1)] != 'A'&&c[min(l + 1, (int)c.size() - 1)])
					{
						ok = false;
						c[l] = 'B';
						break;
					}
				}
				if (ok)
				{
					for (int l = 0;l<c.size(); l++)
					{
						if (c[l] == '#')
						{
							c[l] = 'B';
							break;
						}
					}
				}
				break;
			case'C':
				
				for (int l = 0; l < c.size(); l++)
				{
					if (wan&&c[l] == '#')
					{
						c[l] = 'C';
						ok = false;
						break;
					}
					if (c[l] != '#')
					{
						wan = true;
					}

				}
				if (ok)
				{
					int j = c.size() / 2;
					if (c.size() % 2 == 1)j--;
					c[j] = 'C';
				}
				break;
			case'D':
				if (i == 0)
				{
					c[0] = 'D';
				}
				else
				{
					vector<int> an;
					int j = 150, h = -1;
					for (int l = 0; l < c.size(); l++)
					{
						if (c[l] != '#')an.push_back(l);
					}
					for (int l = c.size() - 1; l >= 0; l--)
					{
						if (c[l] == '#')
						{
							if (j <= min(abs(*lower_bound(an.begin(), an.end(), l) - l), abs(*(lower_bound(an.begin(), an.end(), l) - 1) - l)))
							{
								h = l;
								j = min(abs(*lower_bound(an.begin(), an.end(), l) - l), abs(*(lower_bound(an.begin(), an.end(), l) - 1) - l));
							}

						}
					}
					if (h != -1)
					{
						c[h] = 'D';
					}
					else
					{
						for (int l = 0;l<c.size(); l++)
						{
							if (c[l] == '#')
							{
								c[l] = 'D';
									break;
							}
						}
					}
				}
				break;
			}
		}

		for (int i = 0; i < c.size(); i++)
		{
			cout << c[i];
		}
		cout << endl;

	}
}

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

using namespace std;

int main(){

	int n,m;
	bool en=false;
	while(cin>>n>>m&&!(n==0&&m==0)){


		char chair[101];
		char ch;
		fill(chair,chair+101,'#');
		for(int i = 0; i < m; i++){
			cin>>ch;
			if(ch=='A'){
				for(int j = 0; j < n; j++){
					if(chair[j]=='#'){
						chair[j]=ch;
						break;
					}
				}
			}
			else if(ch=='B'){
				bool f=false;
				for(int j = n-1; j >=0; j--){
					if(chair[j]=='#'&&((j-1<0||chair[j-1]!='A')&&(j+1>=n||chair[j+1]!='A'))){
						chair[j]=ch;
						f=true;
						break;
					}
				}
				// ÀêÈ©Á½ê
				if(!f){
					for(int j = 0; j < n; j++){
						if(chair[j]=='#'){
							chair[j]=ch;
							break;
						}
					}
				}
			}
			else if(ch=='C'){
				bool f=false;
				for(int j = 0; j < n; j++){
					if(chair[j]!='#'){
						f=true;
						break;
					}
				}
				if(!f){
					chair[n/2]==ch;
				}
				else{
					for(int j = 0; j < n; j++){
						if(chair[j]!='#'){
							if(j+1<n&&chair[j+1]=='#'){
								chair[j+1]=ch;
								f=true;
								break;
							}
							else if((j-1>=0)&&chair[j-1]=='#'){
								chair[j-1]=ch;
								f=true;
								break;
							}
						}
					}
				}
			}
			else if(ch=='D'){
				int dists[101];
				fill(dists,dists+101,1000000);
				bool fff=false;
				for(int j = 0; j < n; j++){
					if(chair[j]!='#'){
						fff=true;
						break;
					}
				}
				if(fff==false){
					chair[0]=ch;
				}
				else{
					for(int j = 0; j < n; j++){
						if(chair[j]!='#'){
							for(int k = 0; k < n; k++){
								dists[k]=min(dists[k],abs(j-k));
							}
						}
					}
					int maxCost=-1;
					int idx;
					for(int j = 0; j < n; j++){
						if(chair[j]=='#'){
							if(maxCost<dists[j]){
								maxCost=dists[j];
								idx=j;
							}
						}
					}
					chair[idx]=ch;
				}
			}
		}
		for(int i = 0; i < n; i++){
			cout<<chair[i];
		}
		cout<<endl;
	}

	return 0;
}