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++: //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++){
      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)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>
using namespace std;
typedef long long int lld;

int main(){
  int n,m;
  while(cin >> n >> m , n){
    string str = "";
    for(int i=0;i<n;i++) str += '#';
    for(int c=0;c<m;c++){
      char h; cin >> h;
      if(h == 'A'){
	for(int i=0;i<n;i++){
	  if(str[i] == '#'){
	    str[i] = h;
	    break;
	  }
	}
      } else if(h == 'B'){
	int first = -1;
	bool flg = true;
	for(int i=n-1;i>=0;i--){
	  if(first == -1 && str[i] == '#') first = i;
	  /*
	  if(i == n-1 && str[i] == '#' && str[i-1] != 'A'){
	    str[i] = h;
	    flg = false;
	    break;
	  } else if(i == 0 && str[i] == '#' && str[i+1] != 'A'){
	    str[i] = h;
	    flg = false;
	    break;*/
	  if(str[i] == '#' && str[i-1] != 'A' && str[i+1] != 'A'){
	    str[i] = h;
	    flg = false;
	    break;
	  }
	  if(flg) str[first] = h;
	}
      } else if(h == 'C'){
	if(c == 0) str[(n%2==1 ? (n+1)/2 : (n/2+1)) -1] = 'C';
	else {
	  for(int i=0;i<n;i++){
	    if(str[i] != '#'){
	      if(str[i+1] == '#') {
		str[i+1] = h;
		break;
	      }
	      if(str[i-1] == '#') {
		str[i-1] = h;
		break;
	      }
	    }
	  }
	}
      } else {
	int first = -1;
	int idx = -1;
	int maxx = 0;
	for(int i=0;i<n;i++){
	  if(first == -1 && str[i] == '#') first = i;
	  if(str[i] != '#'){
	    int now = i;
	    int cnt = 0;
	    while(i+1 < n && str[i+1] == '#'){
	      if(first == -1 && str[i+1] == '#') first = i+1;
	      cnt++;
	      i++;
	    }
	    if(i+1 == n) {
	      if(cnt > maxx){
		maxx = cnt;
		idx = now + cnt;
	      }
	    }
	    else if(cnt > 2){
	      cnt = (cnt-1)/2;
	      if(cnt > maxx){
		maxx = cnt;
		idx = now + cnt + 1;
	      }
	    }
	  }
	}
	if(idx == -1) str[first] = h;
	else str[idx] = h;
      }
    }
    cout << str << endl;
  }
}

C++: #include <iostream>
#include <string>

using namespace std;

int main(){
	while(true){
		int n, m;
		cin >> n >> m;
		if(n == 0 && m == 0){ break; }
		char chairs[101];
		for(int i = 0; i < n; i++){ chairs[i] = '#'; }
		chairs[n] = '\0';
		for(int i = 0; i < m; i++){
			string s;
			cin >> s;
			bool flag = false;
			if(s[0] == 'A'){
				for(int j = 0; j < n; j++){
					if(chairs[j] == '#'){ chairs[j] = 'A'; break; }
				}
			}else if(s[0] == 'B'){
				bool flag = false;
				for(int j = n - 1; j >= 0; j--){
					if(chairs[j] != '#'){ continue; }
					if((j < 1 || chairs[j - 1] != 'A') && chairs[j + 1] != 'A'){
						chairs[j] = 'B';
						flag = true;
						break;
					}
				}
				if(!flag){
					for(int j = 0; j < n; j++){
						if(chairs[j] == '#'){ chairs[j] = 'B'; break; }
					}
				}
			}else if(s[0] == 'C'){
				if(i == 0){
					chairs[n / 2] = 'C';
				}else{
					int j = 0;
					for(; j < n; j++){
						if(chairs[j] != '#'){ break; }
					}
					if(chairs[j + 1] == '#'){
						chairs[j + 1] = 'C';
					}else if(j > 0 && chairs[j - 1] == '#'){
						chairs[j - 1] = 'C';
					}else{
						for(; j < n; j++){
							if(chairs[j] == '#'){ break; }
						}
						chairs[j] = 'C';
					}
				}
			}else if(s[0] == 'D'){
				if(i == 0){
					chairs[0] = 'D';
				}else{
					int dist = 0, idx = 0;
					for(int j = 0; j < n; j++){
						if(chairs[j] != '#'){ continue; }
						int d = 100;
						for(int k = j - 1; k >= 0; k--){
							if(chairs[k] != '#'){ d = min(d, j - k); break; }
						}
						for(int k = j + 1; k < n; k++){
							if(chairs[k] != '#'){ d = min(d, k - j); break; }
						}
						if(d > dist){ idx = j; dist = d; }
					}
					chairs[idx] = 'D';
				}
			}
		}
		cout << chairs << endl;
	}
	return 0;
}