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++: #include <cstdio>
#include <cstring>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
	int n, m;
	char str[10001];
	
	while (scanf("%d %d", &n, &m) && n){
		memset(str, '#', sizeof(str));
		vector<int> used;
		str[n] = 0;
		for (int i = 0; i < m; i++){
			char p;
			scanf(" %c", &p);
			
			if (p == 'A'){
				for (int pos = 0; pos < n; pos++){
					if (str[pos] == '#'){
						str[pos] = 'A';
						used.push_back(pos);
						break;
					}
				}
			}
			if (p == 'B'){
				bool seated = false;
				for (int pos = n - 1; pos; pos--){
					if (str[pos] == '#' && (pos == 0 || str[pos - 1] != 'A')){
						str[pos] = 'B';
						used.push_back(pos);
						seated = true; break;
					}
				}
				if (!seated)
					for (int pos = 0; pos < n; pos++){
					if (str[pos] == '#'){
						str[pos] = 'B'; used.push_back(pos);
						break;
					}
				}
			}
			if (p == 'C'){
				if (i == 0){
					if (n & 1){
						str[(n + 1) / 2 - 1] = 'C';
						used.push_back((n + 1) / 2 - 1);
					}
					else {
						str[n / 2] = 'C';
						used.push_back(n / 2);
					}
				}
				else {
					for (int pos = 0; pos < n; pos++){
						if (str[pos] != '#'){
							if (pos + 1 < n && str[pos + 1] == '#'){
								str[pos + 1] = 'C';
								used.push_back(pos + 1);
								break;
							}
							if (pos - 1 >= 0 && str[pos - 1] == '#'){
								str[pos - 1] = 'C';
								used.push_back(pos - 1);
								break;
							}
						}
					}
				}
			}
			if (p == 'D'){
				if (i == 0){
					str[0] = 'D';
					used.push_back(0);
				}
				else {
					sort(used.begin(), used.end());
					int minDist = 999999, minPos = -1;
					for (int pos = 0; pos < n; pos++){
						if (str[pos] == '#'){
							int a = upper_bound(used.begin(), used.end(), pos) - used.begin() - 1;
							int b = upper_bound(used.begin(), used.end(), pos) - used.begin();
							int tDist = 0;
							if (a != -1) tDist = max(tDist, pos - used[a]);
							if (b != used.size()) tDist = max(tDist, used[b] - pos);
							if (tDist < minDist){
								minDist = tDist;
								minPos = pos;
							}
						}
					}
					str[minPos] = 'D';
					used.push_back(minPos);
				}
			}
		}
		printf("%s\n", str);
	}
}

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(s[i-1]!='A'&&s[i]=='#'){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){if(s[i-1]=='#'){s[i-1]='C';pp++;break;}}
            if(i!=n-1){if(s[i+1]=='#'){s[i+1]='C';pp++;break;}}
          }
        if(!pp){
          if(n%2)s[(n+1)/2]='C';
          else s[n/2+1]='C';
        }
      }
      else if(p=="D"){
        int d[n]={},p=0,sp=0;
        for(int i=0;i<n;i++){
          int p1=0,p2=0;
          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;
            sp=max(max(p1,p2),sp);
          p1=min(p1,p2);
          p=max(p,p1);
          d[i]=p1;
        }
        if(sp==n)s[0]='D';
        else for(int i=0;i<n;i++)
          if(d[i]==p){
            s[i]='D';
            break;
          }
      }

    }
    cout<<s<<endl;
  }
}