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[:len(chair)-1], ""))
	}
}

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++: //08
#include<iostream>
#include<string>
#include<algorithm>

using namespace std;

int main(){
  for(int n,m;cin>>n>>m,n|m;){
    string c(n,'#');
    for(int i=0;i<m;i++){
      cout<<c<<endl;
      char p;
      cin>>p;
      switch(p){
      case 'B':
	int j;
	for(j=c.size()-1;j>=0;j--){
	  if(c[j]=='#'&&(j==0||c[j-1]!='A')&&(j==c.size()-1||c[j+1]!='A')){
	    c[j]=p;
	    break;
	  }
	}
	if(j>=0)break;
      case 'A':
	for(int j=0;j<c.size();j++){
	  if(c[j]=='#'){
	    c[j]=p;
	    break;
	  }
	}
	break;
      case 'C':
	if(i==0){
	  c[n/2]=p;
	}else{
	  for(int j=0;j<c.size();j++){
	    if(c[j]!='#'){
	      if(j!=c.size()-1&&c[j+1]=='#'){
		c[j+1]=p;
		break;
	      }else if(j!=0&&c[j-1]=='#'){
		c[j-1]=p;
		break;
	      }
	    }
	  }
	}
	break;
      case 'D':
	if(i==0){
	  c[0]=p;
	}else{
	  int b;
	  int d=-1;
	  for(int j=0;j<c.size();j++){
	    if(c[j]=='#'){
	      int cd;
	      int l;
	      for(l=j;l>=0&&c[l]!='#';l--);
	      cd=(l<0)?1000:j-l;
	      int r;
	      for(r=j;r<n&&c[r]!='#';r++);
	      cd=min(cd,(r==n)?1000:r-j);
	      if(d<cd){
		d=cd;
		b=j;
	      }
	    }
	  }
	  c[b]=p;
	}
	break;
      }
    }
    cout<<c<<endl;
  }
  return 0;
}

C++: #include "bits/stdc++.h"
#include<unordered_map>
#include<unordered_set>
#pragma warning(disable:4996)
using namespace std;
using ld = long double;
const ld eps = 1e-9;

//// < "d:\d_download\visual studio 2015\projects\programing_contest_c++\debug\a.txt" > "d:\d_download\visual studio 2015\projects\programing_contest_c++\debug\b.txt"

void sit(vector<int>&seats, int who) {
	if (who == 1) {
		for (int i = 0; i < seats.size(); ++i) {
			if (!seats[i]) {
				seats[i] = who;
				return;
			}
		}
	}
	else if (who == 2) {
		for (int i = seats.size()-1; i >= 0; --i) {
			if (!seats[i]) {
				bool ok = true;
				for (int k = i+1; k >= i-1; k -= 2) {
					if (k >= 0 && k < seats.size()) {
						if (seats[k] == 1)ok = false;
					}
				}
				if (ok) {
					seats[i] = who;
					return;
				}
			}
		}
		for (int i = 0; i < seats.size(); ++i) {
			if (!seats[i]) {
				seats[i] = who;
				return;
			}
		}
	}
	else if (who == 3) {
		for (int i = 1; i < seats.size(); ++i) {
			if (seats[i - 1]) {
				if (!seats[i]) {
					seats[i] = who;
					return;
				}
			}
		}
		seats[seats.size() / 2] = who;
		return;
	}
	else {
		vector<int>diss(seats.size(), 1000);
		for (int i = 0; i < seats.size(); ++i) {
			if (seats[i]) {
				diss[i] = 0;
				for (int j = i - 1; j >= 0; --j) {
					diss[j] = min(diss[j], i-j);
				}
				for (int j = i + 1; j < seats.size(); ++j) {
					diss[j] = min(diss[j], j - i);
				}
			}
		}
		int maxdis = -1;
		int ans = -1;
		for (int i = 0; i < seats.size(); ++i) {
			if (maxdis < diss[i]) {
				maxdis = diss[i];
				ans = i;
			}
		}
		seats[ans] = who;
		return;
	}
	assert(false);
}

int main() {
	while (1) {
		int N, M; cin >> N >> M;
		if (!N)break;
		vector<int>seats(N);
		for (int i = 0; i < M; ++i) {
			char c; cin >> c;
			int num = c - 'A' + 1;
			sit(seats, num); 
			
		}
		for (int i = 0; i < N; ++i) {
			if (seats[i]) {
				cout << char('A' + seats[i] - 1);
			}
			else {
				cout << '#';
			}
		}
		cout << endl;
	}
	return 0;
}

C++: #include <cstdio>
#include <string>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <iostream>

using namespace std;

int n,m;
int fie[101];
int dist[101];
char data[5]={'#','A','B','C','D',};

int main(void){
	while(1){
		scanf("%d %d",&m,&n);
		if(n+m==0)break;
		memset(fie,0,sizeof(fie));
		for(int i=0;i<n;i++){
			string a;
			cin >> a;
			if(a[0]=='A'){
				for(int j=0;j<m;j++){
					if(fie[j]==0){
						fie[j]=1;
						break;
					}
				}
			}
			if(a[0]=='B'){
				int j;
				for(j=m-1;j>=0;j--){
					if(j>0 && fie[j-1]==1)continue;
					if(j<m-1 && fie[j+1]==1)continue;
					if(fie[j]==0){
						fie[j]=2;
						break;
					}
				}
				if(j==-1){
					for(int j=0;j<m;j++){
						if(fie[j]==0){
							fie[j]=2;
							break;
						}
					}
				}
			}
			if(a[0]=='C'){
				if(i==0){
					fie[m/2]=3;
				}else{
					for(int j=0;j<m;j++){
						if(fie[j]!=0){
							if(j<m-1 && fie[j+1]==0){
								fie[j+1]=3;
								break;
							}
							if(j>0 && fie[j-1]==0){
								fie[j-1]=3;
								break;
							}
						}
					}
				}
			}
			if(a[0]=='D'){
				if(i==0){
					fie[0]=4;
				}else{
					for(int j=0;j<m;j++)dist[j]=101;
					for(int j=0;j<m;j++){
						if(fie[j]!=0){
							for(int k=0;k<m;k++){
								dist[k]=min(dist[k],abs(k-j));
							}
						}
					}
					int lll=0;
					for(int j=0;j<m;j++){
						if(dist[j]>dist[lll])lll=j;
					}
					fie[lll]=4;
				}
			}
		}
		for(int i=0;i<m;i++){
			printf("%c",data[fie[i]]);
		}
		printf("\n");
	}
	return 0;
}