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++: // 2011/08/02 Tazoe

#include <iostream>
using namespace std;

int main()
{
	while(true){
		int n, m;
		cin >> n >> m;
		if(n==0&&m==0)
			break;

		char C[10000];
		for(int i=0; i<n; i++)
			C[i] = '#';

		for(int i=0; i<m; i++){
			char a;
			cin >> a;

			if(a=='A'){
				for(int j=0; j<n; j++)
					if(C[j]=='#'){
						C[j] = a;
						break;
					}
			}
			else if(a=='B'){
				bool flg = false;
				for(int j=n-1; j>=0; j--)
					if(C[j]=='#'&&(j==0||C[j-1]!='A')&&(j==n-1||C[j+1]!='A')){
						C[j] = a;
						flg = true;
						break;
					}
				if(!flg)
					for(int j=0; j<n; j++)
						if(C[j]=='#'){
							C[j] = a;
							break;
						}
			}
			else if(a=='C'){
				if(i==0){
					C[n/2] =a;
				}
				else{
					for(int j=0; j<n; j++)
						if(C[j]!='#'&&j!=n-1&&C[j+1]=='#'){
							C[j+1] = a;
							break;
						}
						else if(C[j]!='#'&&j!=0&&C[j-1]=='#'){
							C[j-1] = a;
							break;
						}
				}
			}
			else if(a=='D'){
				if(i==0){
					C[0] = a;
				}
				else{
					int D[10000];
					int d = 10000;
					for(int j=0; j<n; j++)
						if(C[j]!='#'){
							d = 0;
							D[j] = d;
						}
						else{
							d++;
							D[j] = d;
						}
					d = 10000;
					for(int j=n-1; j>=0; j--)
						if(C[j]!='#'){
							d = 0;
							continue;
						}
						else{
							d++;
							if(D[j]>d)
								D[j] = d;
						}
					int max_d = -1;
					int max_j = -1;
					for(int j=0; j<n; j++)
						if(D[j]>max_d){
							max_d = D[j];
							max_j = j;
						}
					C[max_j] = a;
				}
			}
		}

		for(int i=0; i<n; i++)
			cout << C[i];
		cout << endl;
	}

	return 0;
}

C++: #include<bits/stdc++.h>
#define rep(i,n)for(int i=0;i<n;i++)
using namespace std;

char s[200], a[2];
int main() {
	bool b;
	int n, m;
	while (scanf("%d%d", &n, &m), n) {
		memset(s, 0, sizeof(s)); rep(i, n)s[i] = '#';
		rep(i, m) {
			scanf("%s", a);
			switch (a[0]) {
			case 'A':
				rep(i, n) { if (s[i] == '#') { s[i] = 'A'; break; } }
				break;
			case 'B':
				b = false;
				for (int i = n - 1; i >= 0; i--) {
					if (s[i] == '#' && (!i || s[i - 1] != 'A') && (i + 1 >= n || s[i + 1] != 'A')) {
						b = true; s[i] = 'B'; break;
					}
				}
				if (!b)rep(i, n) { if (s[i] == '#') { s[i] = 'B'; break; } }
				break;
			case 'C':
				b = false;
				rep(i, n) {
					if (s[i] != '#') {
						if (i + 1 < n&&s[i + 1] == '#') { s[i + 1] = 'C'; b = true; break; }
						if (i&&s[i - 1] == '#') { s[i - 1] == 'C'; b = true; break; }
					}
				}
				if (!b)s[n / 2] = 'C';
				break;
			case 'D':
				int k, Max = -1;
				rep(i, n) {
					if (s[i] != '#')continue;
					int cnt = INT_MAX;
					int t = i - 1;
					for (; t >= 0 && s[t] == '#'; t--);
					cnt = t == -1 ? INT_MAX : i - t;
					t = i + 1;
					for (; t < n&&s[t] == '#'; t++);
					cnt = min(cnt, t == n ? INT_MAX : t - i);
					if (Max < cnt) {
						k = i; Max = cnt;
					}
				}
				s[k] = 'D';
				break;
			}
		}
		printf("%s\n", s);
	}
}

C++: #include<bits/stdc++.h>
using namespace std;
int main(){
  int n,m;
  while(cin>>n>>m,n){
    string s;
    for(int i=0;i<n;i++)s+='#';
    while(m--){
      string p;
      cin>>p;
      if(p=="A"){
        for(int i=0;i<n;i++)
          if(s[i]=='#'){
            s[i]='A';
            break;
          }
      }
      if(p=="B"){
        int pp=0;
        for(int i=n-1;i>=0;i--){
          if(i==n-1){if(s[i-1]!='A'&&s[i]=='#'){pp++,s[i]='B';break;}}
          else if(!i){if(s[i+1]!='A'&&s[i]=='#'){pp++,s[i]='B';break;}}
          else {if(s[i-1]!='A'&&s[i]=='#'&&s[i+1]!='A'){pp++,s[i]='B';break;}}
        }
        if(!pp)for(int i=0;i<n;i++)
             if(s[i]=='#'){
               s[i]='B';
               break;
            }
      }
      if(p=="C"){
        int pp=0;
        for(int i=0;i<n;i++)
          if(s[i]!='#'){
            
            if(i!=n-1){if(s[i+1]=='#'){s[i+1]='C';pp++;break;}}
            if(i){if(s[i-1]=='#'){s[i-1]='C';pp++;break;}}
          }
        if(!pp){
          if(n%2)s[(n+1)/2-1]='C';
          else s[n/2]='C';
        }
      }
      else if(p=="D"){
        int d[n]={},p=0,sp=0;
        for(int i=0;i<n;i++){

          int p1=0,p2=0;
          if(i==n-1)p1=1000;
          if(i==0)p2=1000;
          for(int j=i;j<n;j++)
            if(s[j]=='#')p1++;
            else break;
          for(int j=i;j>=0;j--)
            if(s[j]=='#')p2++;
            else break;
          p1=min(p1,p2);
          p=max(p,p1);
          d[i]=p1;
        }
        for(int i=0;i<n;i++)if(s[i]!='#')sp++;
        if(sp)s[0]='D';
        else for(int i=0;i<n;i++)
          if(d[i]==p){
            s[i]='D';
            break;
          }
      }

    }
    cout<<s<<endl;
  }
}