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, ""))
	}
}

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<iostream>
#include<algorithm>
using namespace std;

int main(){
  int n,m;

  for(;;){
    cin >> n >> m;
    if(!n && !m)break;

    string c;
    for(int i=0;i<n;i++)c += '#';

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

      int pos;
      if(g == 'A'){
	pos = 0;
	while(c[pos]!='#')pos++;
      }else if(g == 'B'){
	pos = n-1;
	while(pos>=0){
	  if(c[pos]=='#'){
	    if( (pos==n-1 || c[pos+1]!='A') && (pos==0 || c[pos-1]!='A') )break;
	  }
	  pos--;
	}
	if(pos<0){
	  pos = 0;
	  while(c[pos]!='#')pos++;
	}
      }else if(g == 'C'){
	if(!i)pos = n/2;
	else{
	  pos = 0;
	  while(pos<n){
	    if(c[pos]!='#'){
	      if(pos<n-1 && c[pos+1]=='#'){
		pos++;
		break;
	      }
	      if(pos>0 && c[pos-1]=='#'){
		pos--;
	      break;
	      }
	    }
	    pos++;
	  }
	}
      }else{
	pos = 0;
	if(i){
	  int dis = 0;
	  int tmp = -1;
	  while(pos<n){
	    if(c[pos]=='#'){
	      int l = 1,r = 1;
	      while(pos-l>=0 && c[pos-l]!='#')l++;
	      while(pos+r<n && c[pos+r]!='#')r++;
	      if(pos-l<0){
		if(dis<r){
		  dis = r;
		  tmp = pos;
		}
	      }else if(pos+r>=n){
		if(dis<l){
		  dis = l;
		  tmp = pos;
		}
	      }else{
		if(dis<min(l,r)){
		  dis = min(l,r);
		  tmp = pos;
		}
	      }
	    }
	    pos++;
	  }
	  pos = tmp;
	}
      }
      c[pos] = g;
    }

    cout << c << endl;
  }
}

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

using namespace std;

int n, m;
char ch[102];

void funcA()
{
  for(int i = 1; i <= n; ++i)
    if(ch[i] == '#'){
      ch[i] = 'A';
      break;
    }

  return ;
}

void funcB()
{
  for(int i = n; i > 0; --i)
    if(ch[i] == '#' && ch[i-1] != 'A' && ch[i+1] != 'A'){
      ch[i] = 'B';
      return ;
    }

  for(int i = 1; i <= n; ++i)
    if(ch[i] == '#'){
      ch[i] = 'B';
      break;
    }

  return ;
}

void funcC()
{
  bool f = true;
  for(int i = 1; i <= n && f; ++i)
    if(ch[i] != '#') f = false;

  if(f){
    int t = (n%2 == 0)? (n/2+1): ((n+1)/2);
    ch[t] = 'C';
    return ;
  }

  for(int i = 1; i <= n; ++i)
    if(ch[i] != '#'){
      if(i+1 <= n && ch[i+1] == '#'){
	ch[i+1] = 'C';
	break;
      }
      if(i-1 > 0 && ch[i-1] == '#'){
	ch[i-1] = 'C';
	break;
      }
    }

  return ;
}

void funcD()
{
  int a[102], p;
  fill(a, a+102, 0);

  for(int i = 1; i <= n; ++i){
    if(ch[i] == '#'){
      p = i-1;
      while(0 < p && ch[p] == '#'){
	a[i]++;
	--p;
      }
      if(p == 0) a[i] = 0;

      p = i+1;
      while(p <= n && ch[p] == '#'){
	a[i]++;
	++p;
      }
      if(p > n) a[i] -= (p-i-1);
    } else {
      a[i] = -1;
    }
  }

  p = 1;
  for(int i = 2; i <= n; ++i)
    if(a[i] > a[p]) p = i;
  ch[p] = 'D';

  return ;
}

int main()
{
  char p;

  while(cin>>n>>m && (n+m)){
    fill(ch, ch+102, '#');
    while(m--){
      cin >> p;
      if(p == 'A') funcA();
      if(p == 'B') funcB();
      if(p == 'C') funcC();
      if(p == 'D') funcD();
    }

    for(int i = 1; i <= n; ++i) cout << ch[i];
    cout << endl;
  }

  return 0;
}

C++: #include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

int chair[100],pos,n,m;

int d(char c){
	if(c == 'A'){
		for(int i=0;i<n;i++){
			if(chair[i] == -1) return i;
		}
	}else if(c == 'B'){
		for(int i=n-1;i>=0;i--){
			if(chair[i] == -1){
				if(i == 0){
					if(chair[i+1] != 'A') return i;
				}else if(i == n-1){
					if(chair[i-1] != 'A') return i;
				}else{
					if(chair[i-1] != 'A' && chair[i+1] != 'A') return i;
				}
			}
		}
		for(int i=0;i<n;i++){
			if(chair[i] == -1) return i;
		}
	}else if(c == 'C'){
		for(int i=0;i<n;i++){
			if(chair[i] != -1){
				if(i+1 < n && chair[i+1] == -1) return i;
				if(0 <= i-1 && chair[i-1] == -1) return i;
			}
		}
		return n/2+1;
	}else{
		int d_min_max = 0,d_pos=-1,tmp;
		for(int i=0;i<n;i++){
			//ó¢Ä¢éÖqÆÅàß¢lÌ£ðßé
			
			if(chair[i] == -1){
				int dist_min = 1 << 28;
				if(i != n-1){
					tmp = 1;
					while(i+tmp < n && chair[i+tmp] == -1) tmp++;
					if(i+tmp<n){
						dist_min = min(dist_min, tmp);
					}
				}
				if(i != 0){
					tmp = 1;
					while(i-tmp >= 0 && chair[i-tmp] == -1) tmp++;
					if(i-tmp >= 0){
						dist_min = min(dist_min, tmp);
					}
				}
				if(d_min_max < dist_min){
					d_min_max = dist_min;
					d_pos = i;
				}
			}
		}
		if(d_pos != -1) return d_pos;
		for(int i=0;i<n;i++){
			if(chair[i] == -1) return i;
		}
	}
}

int main(){
	while(cin >> n >> m){
		if(!(n|m)) break;
		char c;
		memset(chair, -1, sizeof(chair));
		for(int i=0;i<m;i++){
			cin >> c;
			chair[d(c)] = c;
		
		for(int i=0;i<n;i++){
			if(chair[i] == -1){
				cout << "#";
			}else{
				cout << (char)chair[i];
			}
		}
		cout << endl;
	}
	}
}