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<iostream>
#include<cstring>
#include<algorithm>
#include<vector>
#include<map>

using namespace std;

int N, M;
char ChairsPerson[101];
int Offset[101];


void solveA() {

	for (int i = 1; i <= N; ++i) {
		if (ChairsPerson[i]=='#') {
			ChairsPerson[i] = 'A';
			return;
		}
	}
}

void solveB() {

	for (int i = N; i >= 1; --i) {
		if (ChairsPerson[i] != '#') {
			continue;
		}
		if (i > 1 && ChairsPerson[i - 1] == 'A') {
			i -= 2;
			continue;
		}
		if (i < N  && ChairsPerson[i + 1] == 'A') {
			continue;
		}
		ChairsPerson[i] = 'B';
		return;
	}

	for (int i = 1; i <= N; ++i) {
		if (ChairsPerson[i] == '#') {
			ChairsPerson[i] = 'B';
			return;
		}
	}
}

void solveC(int id) {

	if (id == 0) {
		//firstperson
		ChairsPerson[N / 2 + 1] = 'C';
		return;
	}

	for (int i = 1; i <= N; ++i) {
		if (ChairsPerson[i] == '#') {
			continue;
		}
		if (i < N && ChairsPerson[i + 1] == '#') {
			ChairsPerson[i + 1] = 'C';
			return;
		}
		if (i > 1 && ChairsPerson[i - 1] == '#') {
			ChairsPerson[i - 1] = 'C';
			return;
		}
	}	
}

void solveD() {
	int pos, pos1, pos2, at;

	for (pos1 = 1; pos1 <= N; ++pos1) {
		if (ChairsPerson[pos1] == '#') {
			continue;
		}
		for (pos2 = 1; pos2 <= N; ++pos2) {
			Offset[pos2] = min(Offset[pos2], abs(pos1 - pos2));
		}
	}

	pos = 0;
	at = 1;
	for (pos1 = 1; pos1 <= N; ++pos1) {
		//cout << "pos1:" << pos1 << "  " << Offset[pos1] << endl;
		if (Offset[pos1] > pos) {
			pos = Offset[pos1];
			at = pos1;
		}
	}
	ChairsPerson[at] = 'D';
}

int main() {
	char person;

	while (cin >> N >> M, N) {
		fill(ChairsPerson + 1, ChairsPerson + N + 1, '#');
		fill(Offset + 1, Offset + N + 1, 1000);

		for (int i = 0; i < M; ++i) {
			cin >> person;
			switch (person)
			{
			case 'A':solveA(); break;
			case 'B':solveB(); break;
			case 'C':solveC(i); break;
			case 'D':solveD(); break;
			default:break;
			}
		}
		for (int i = 1; i <= N; ++i) {
			cout << ChairsPerson[i];
		}
		cout << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <math.h>
#include <vector>
#include <set>

using namespace std;
typedef long long LL;
static const double EPS = 1e-9;

#define FOR(i,k,n) for (int i=(k); i<(int)(n); ++i)
#define REP(i,n) FOR(i,0,n)
#define P(x) cout << (x) << endl

int main(void){  
  int n,m;
  while(cin>>n>>m){
    if(!n) break;
    int ch[102] = {0};
    char a;
    REP(h,m){
      cin >> a;
      if(a == 'A'){
        FOR(i,1,n+1){
          if(ch[i] == 0){
            ch[i] = 'A';
            break;
          }
        }
      }else if(a == 'B'){
        bool f=false;
        for(int i=n; i>0; i--){
          if(ch[i]==0 && ch[i+1] != 'A' && ch[i-1] != 'A'){
            ch[i] = 'B';
            f = true;
            break;
          }
        }
        if(!f){
          FOR(i,1,n+1){
            if(ch[i] == 0){
              ch[i] == 'B';
              break;
            }
          }
        }
      }else if(a == 'C'){
        int c=-1;
        FOR(i,1,n+1){
          if(ch[i] != 0){
            c = i;
            break;
          }
        }
        if(c==-1){
          ch[(n%2)?(n+1)/2:n/2+1] = 'C';
        }else{
          while(c<n+1){
            if(ch[c+1]==0&&c<n){
              ch[c+1] = 'C';
              break;
            }else if(ch[c-1]==0&&c>1){
              ch[c-1] = 'C';
              break;
            }
            c++;
          }
        }
      }else if(a == 'D'){
        int maxi,max=0;
        FOR(i,1,n+1){
          if(ch[i] == 0){
            int l=100,r=100,mi;
            for(int j=i-1; j>0; j--)
              if(ch[j] != 0){
                l = i-j;
                break;
              }
            for(int j=i+1; j<n+1; j++)
              if(ch[j] != 0){
                r = j-i;
                break;
              }
            mi = min(l,r);
            if(mi>max){
              maxi = i;
              max = mi;
            }
          }
        }
        ch[maxi] = 'D';
      }
    }
    for(int i=1; i<n+1; i++){
      if(ch[i] == 0) cout <<"#";
      else printf("%c",ch[i]);
    }
    cout << endl;
  }
  return 0;
}