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;

int main(){

  while(1){

    int n,m;

    cin >> n >> m;

    if(n==0 && m == 0) break;

    char ch[n];

    for(int i=0;i<n;i++){
      ch[i] = '#';
    }

    for(int k=0;k<m;k++){
      char c;

      cin >> c;

      if(c=='A'){
        for(int i=0;i<n;i++){
          if(ch[i]=='#'){
            ch[i] = 'A';
            break;
          }
        }
      }
      else if(c == 'B'){
        bool flag = true;
        for(int i=n-1;i!=0;i--){
          if(ch[i]=='#'){
            if(i == n-1 && ch[i-1]!='A'){
              ch[i] = 'B';
              flag = false;
              break;
            }
            else if(i == 0 && ch[i+1]!='A'){
              ch[i] = 'B';
              flag = false;
              break;
            }
            if((ch[i+1]!='A') && (ch[i-1]!='A')){
              ch[i] = 'B';
              flag = false;
              break;
            }
          }
        }
        if(flag){
          for(int i=0;i<n;i++){
            if(ch[i]=='#'){
              ch[i] = 'B';
              break;
            }
          }
        }
      }
      else if(c == 'C'){
        if(k==0){
          if(n%2==0){
            ch[n/2+1] = 'C';
          }
          else{
            ch[(n+1)/2] = 'C';
          }
        }
        else{
          for(int i=0;i<n;i++){
            if(ch[i]!='#'){
              if(ch[i+1]=='#'){
                ch[i+1] = 'C';
                break;
              }
              else if(i!=0 && ch[i-1]=='#'){
                ch[i-1] = 'C';
                break;
              }
              else{
                for(int j=i;j<n;j++){
                  if(ch[j]=='#'){
                    ch[j] = 'C';
                    break;
                  }
                }
              }
            }
          }
        }
      }
      else{
        int dis[n];
        int mdis = 0,pos = 0;
        if(k==0){
          ch[0] = 'D';
        }
        else{
          for(int i=0;i<n;i++){
            if(ch[i]!='#'){
              dis[0] = i;
              break;
            }
          }
          for(int i=1;i<n;i++){
            if(ch[i] != '#'){
              dis[i] = 0;
              continue;
            }
            dis[i] = dis[i-1] + 1;
            for(int j=i+1;j<n;j++){
              if(ch[j] != '#'){
                dis[i] = min(dis[i],j-i);
              }
            }
          }
          for(int i=0;i<n;i++){
            if(dis[i] > mdis){
              mdis = dis[i];
              pos = i;
            }
          }
          ch[pos] = 'D';
        }
      }
      for(int i=0;i<n;i++){
        cout << ch[i];
      }
      cout << endl;
    }





  }
}

C++: #include<stdio.h>
#include<string.h>
#include<math.h>
int main(void)
{
	int a,s,d[101],f,g,h,i,j,aa,k,ss,dd,ff,gg,xx,cc;
	char z[2];
	while(1){
		scanf("%d %d",&a,&s);
		if(a==0&&s==0) break;
		for(i=0;i<a;i++) d[i]=0;
		for(i=0;i<s;i++){
			scanf("%s",z);
			if(z[0]=='A'){
				for(j=0;j<a;j++){
					if(d[j]==0){
						d[j]=1;
						break;
					}
				}
			}
			else if(z[0]=='B'){
				aa=0;
				for(j=a-1;j>=0;j--){
					if(d[j]==0){
						if(j==a-1){
							if(d[j-1]!=1){
								d[j]=2;
								aa=1;
								break;
							}
						}
						else if(j==0){
							if(d[1]!=1){
								d[j]=2;
								aa=1;
								break;
							}
						}
						else{
							if(d[j-1]!=1&&d[j+1]!=1){
								d[j]=2;
								aa=1;
								break;
							}
						}
					}
				}
				if(aa==0){
					for(j=0;j<a;j++){
						if(d[j]==0){
							d[j]=2;
							break;
						}
					}
				}
			}
			else if(z[0]=='C'){
				aa=0;
				for(j=0;j<a;j++){
					if(d[j]!=0){
						aa=1;
						break;
					}
				}
				if(aa==0) d[a/2]=3;
				else if(aa==1){
					for(j=0;j<a;j++){
						if(d[j]!=0){
							if(j!=0&&d[j-1]==0){
								d[j-1]=3;
								break;
							}
							else if(d[j+1]==0){
								d[j+1]=3;
								break;
							}
						}
					}
				}
			}
			else if(z[0]=='D'){
				aa=0;
				ss=0;
				ff=0;
				gg=0;
				for(j=0;j<a;j++){
					if(d[j]!=0){
						gg=1;
						break;
					}
				}
				if(gg==0) d[0]=4;
				else{
					for(j=a-1;j>=0;j--){
						dd=0;
						xx=0;
						if(d[j]==0){
							ss=j;
							for(k=j;k>=0;k--){
								if(k==a-1) xx=2;
								if(d[k]!=0){
									break;
								}
								if(k==0) xx=1;
								dd++;
							}
						}
						printf("%d  %d\n",j,dd);
						if(xx==2&&aa<=dd*2){
							aa=dd*2;
							ff=ss;
							cc=1;
						}
						if(xx==1&&aa<=dd*2){
							aa=dd*2;
							ff=ss;
							cc=2;
						}
						if(xx==0){
							if(aa<=dd){
								aa=dd;
								ff=ss;
								cc=0;
							//	printf("%d %d %d\n\n",ff,dd,dd/2);
							}
						}
					}
				//	printf("%d %d %d\n\n",aa,dd,cc);
					if(cc==2) d[0]=4;
					if(cc==1) d[aa/2]=4;
					if(cc==0) d[ff-(aa/2)]=4;
				}
			}
			/*for(j=0;j<a;j++){
				if(d[j]==0) printf("#");
				else if(d[j]==1) printf("A");
				else if(d[j]==2) printf("B");
				else if(d[j]==3) printf("C");
				else if(d[j]==4) printf("D");
			}
			printf("\n");*/
		}
		for(i=0;i<a;i++){
			if(d[i]==0) printf("#");
			else if(d[i]==1) printf("A");
			else if(d[i]==2) printf("B");
			else if(d[i]==3) printf("C");
			else if(d[i]==4) printf("D");
		}
		printf("\n");
	}
	return 0;
}