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<iostream>
#include<string>
using namespace std;
int main(){
  int n,m;
  while(cin >>n>>m,n||m){
    int row[100] = {0},chair[102] = {0};
    for(int i=0; i<m; i++){
      string s;
      cin >>s;
      if(s == "A"){row[i] = 1;}
      if(s == "B"){row[i] = 2;}
      if(s == "C"){row[i] = 3;}
      if(s == "D"){row[i] = 4;}
    }
    bool flag = true;
    for(int i=0; i<m; i++){
      if(row[i] == 1){
	for(int j=0; j<n; j++){
	  if(!chair[j+1]){
	    chair[j+1] = 1;
	    break;
	  }
	}
      }
      if(row[i] == 2){
	bool non = true;
	for(int j=n-1; j>=0; j--){
	  if(chair[j] != 1 && chair[j+1] == 0 && chair[j+2] != 1){
	    chair[j+1] = 2;
	    non = false;
	    break;
	  }
	}
	if(non){
	  for(int j=0; j<n; j++){
	    if(!chair[j+1]){
	      chair[j+1] = 2;
	      break;
	    }
	  }
	}
      }
      if(row[i] == 3){
	if(flag){
	  if(n%2){chair[(n+1)/2] = 3;}
	  else{chair[n/2+1] = 3;}
	}
	else{
	  for(int j=1; j<n; j++){
	    if(chair[j] && !chair[j+1]){
	      chair[j+1] = 3;
	      break;
	    }
	  }
	}
      }
      if(row[i] == 4){
	if(flag){chair[1] = 4;}
	else{
	  int r = 0,s = -1;
	  bool non = true;
	  for(int j=0; j<n; j++){
	    if(!chair[j+1]){
	      int x = 200;
	      for(int k=j+1; k<n; k++){
		if(chair[k+1]){
		  if(x > k-j){x = k-j;}
		  break;
		}
	      }
	      for(int k=j-1; k>=0; k--){
		if(chair[k+1]){
		  if(x > j-k){x = j-k;}
		  break;
		}
	      }
	      if(r < x){r = x;s = j;}
	    }
	  }
	  if(s != -1){
	    chair[s+1] = 4;
	    non = false;
	  }
	  if(non){
	    for(int j=0; j<n; j++){
	      if(!chair[j+1]){
		chair[j+1] = 4;
		break;
	      }
	    }
	  }
	}
      }
      flag = false;
    }
    for(int i=0; i<n; i++){
      if(chair[i+1] == 0){cout <<"#";}
      if(chair[i+1] == 1){cout <<"A";}
      if(chair[i+1] == 2){cout <<"B";}
      if(chair[i+1] == 3){cout <<"C";}
      if(chair[i+1] == 4){cout <<"D";}
    }
    cout <<endl;
  }
  return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <map>
#include <set>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <string>
 
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define FFOR(i,a,b) for(int i=(a);i<=(b);i++)
#define REP(i,b) FOR(i,0,b)
#define RREP(i,b) FFOR(i,1,b)
#define PB push_back
#define F first
#define S second
#define BE(c) c.begin(),c.end()
using namespace std;
typedef long long LL;
typedef LL ut;
typedef long double ld;
typedef pair<ut,ut> pr;
typedef vector<pr> Vpr;
typedef vector<ut> VI;
typedef pair<ut,pr> ppr;
typedef priority_queue<pr,Vpr, greater<pr> > PQ;
const int SIZE=5+3*1e+4;
const ut INF=1<<30;
const ld eps=1e-6;
const LL p=7+1e+9;

int main(){
	int n, m;
	
	while(cin >> n >> m && (n || m)){
		char c;
		int state[102] = {0};
		state[0] = state[n + 1] = -1;
		REP(i,m){
			cin >> c;
			if(c == 'A'){
				RREP(j,n){
					if(state[j] == 0){
						state[j] = 1;
						break;
					}
				}
			}
			else if(c == 'B'){
				bool ok = false;
				for(int j = n; j > 0; --j){
					if(state[j] == 0 && state[j + 1] != 1 && state[j - 1] != 1){
						state[j] = 2;
						ok = true;
						break;
					}
				}
				if(!ok){
					RREP(j,n){
						if(state[j] == 0){
							state[j] = 2;
							break;
						}
					}
				}
			}
			else if(c == 'C'){
				if(i){
					RREP(j,n){
						if(state[j] != 0){
							if(state[j + 1] == 0){
								state[j + 1] = 3;
								break;
							}
							else if(state[j - 1] == 0){
								state[j - 1] = 3;
								break;
							}
						}
					}
				}
				else{
					if(n % 2){
						state[(n + 1) / 2] = 3;
					}
					else{
						state[n / 2 + 1] = 3;
					}
				}
			}
			else{
				if(i){
					int left = 500, right, tmp;
					RREP(j,n){
						if(state[j] != 0){
							right = j;
							break;
						}
					}
					int maxium = 0, pos = 0;
					RREP(j,n){
						if(state[j] == 0){
							int t = min(j - left, right - j);
							if(maxium < t){
								maxium = t;
								pos = j;
							}
						}
						if(right <= j){
							left = right;
							bool discovered = false;
							FFOR(k,j + 1,n){
								if(state[k] != 0){
									right = k;
									discovered = true;
									break;
								}
							}
							if(!discovered){
								right = 500;
							}
						}
					}
					state[pos] = 4;
				}
				else{
					state[1] = 4;
				}
			}
		}
		RREP(i,n){
			if(state[i] == 0)
				cout << "#";
			else if(state[i] == 1)
				cout << "A";
			else if(state[i] == 2)
				cout << "B";
			else if(state[i] == 3)
				cout << "C";
			else if(state[i] == 4)
				cout << "D";
		}
		cout << endl;
	}
}