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<bits/stdc++.h>
using namespace std;
typedef long long int lld;
#define INF 100000
int main(){
  int n,m;
  while(cin >> n >> m , n || m){
    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(str[i] == '#' && str[i-1] != 'A' && str[i+1] != 'A'){
	    str[i] = h;
	    flg = false;
	    break;
	  }
	  if(flg){
	    for(int i=0;i<n;i++){
	      if(str[i] == '#'){
		str[i] = h;
		break;
	      }
	    }	
	  }
	}
      } 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 k = (c==0 ? 0 : 1);
	int MAX=0,j,m,MAXi;
	for(int i=0;i<n;i++){
	  if(str[i]=='#'){
	    for(j=i-1;j>=0;j--) if(str[j]!='#') break;
	    for(m=i+1;m<n;m++) if(str[m]!='#') break;
	    if(j==-1) j = INF;
	    if(m==n) m = INF;
	    if(j==INF){
	      if(k-i > MAX) MAX = m-i,MAXi=i,MAXi=i;
	    } else if(m==INF){
	      if(i-j > MAX) MAX = i-j,MAXi=i;
	    } else{
	      if(min(m-i,i-j) > MAX) MAX=min(m-i,i-j),MAXi=i;
	    }
	  }
	}
	if(MAX == 0) {
	  for(int i=0;i<n;i++){
	    if(str[i] == '#'){
	      str[i] = h;
	      break;
	    }
	  }
	}
	else str[MAXi] = h;
      }
    }
    cout << str << endl;
  }
}

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

#define int long long

using namespace std;

class Solver {
};

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

		vector<char> status(n, '#');
		for (int i = 0; i < m; i++) {
			char who;
			cin >> who;
			if (who == 'A') {
				for (int i = 0; i < n; i++) {
					if (status[i] == '#') {
						status[i] = 'A';
						break;
					}
				}
			}
			if (who == 'B') {
				bool valid = false;
				for (int i = n - 1; i >= 0; i--) {
					if (status[i] != '#')continue;

					if ((i + 1 < n && status[i + 1] == 'A')
						|| (i - 1 >= 0 && status[i - 1] == 'A'))continue;

					status[i] = 'B';
					valid = true;
					break;
				}

				if (!valid) {
					for (int i = 0; i < n; i++) {
						if (status[i] == '#') {
							status[i] = 'B';
							break;
						}
					}
				}
			}
			if (who == 'C') {
				bool valid = false;
				for (int i = 0; i <n; i++) {
					if (status[i] == '#')continue;

					if (i + 1 < n && status[i + 1] == '#') {
						status[i + 1] = 'C';
						valid = true;
						break;
					}
					if (i - 1 >= 0 && status[i - 1] == '#') {
						status[i - 1] = 'C';
						valid = true;
						break;
					}
				}

				if (!valid) {
					status[n / 2] = 'C';
				}
			}
			if (who == 'D') {
				vector<int> left(n, 0);
				int last = -1;
				for (int i = 0; i < n; i++) {
					if (status[i] != '#') last = i;
					else {
						if (last >= 0) left[i] = i - last;
						else left[i] = 1e9;
					}
				}
				last = -1;
				vector<int> right(n, 0);
				for (int i = n - 1; i >= 0; i--) {
					if (status[i] != '#')last = i;
					else {
						if (last >= 0) right[i] = last - i;
						else right[i] = 1e9;
					}
				}
				int now = 0;
				int dist = -1;
				for (int i = 0; i < n; i++) {
					if (status[i] != '#')continue;

					if (min(left[i], right[i]) > dist) {
						now = i;
						dist = min(left[i], right[i]);
					}
				}
				status[now] = 'D';
			}
		}

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