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<stdio.h>
int main(void)
{
    int n,m,i,j,flg,w,d,p,q,k,e[100];
	char s;
	while(1){
		scanf("%d %d",&n,&m);
		if(n==0&&m==0) break;
		for(i=0;i<n;i++) e[i]=0;
		for(i=0;i<m;i++){
			scanf(" %c",&s);
			if(s=='A'){
				for(j=0;j<n;j++){
					if(e[j]==0){
						e[j]=1; 
						break;
					}
				}
			}
			else if(s=='B'){
				flg=0;
				for(j=n-1;j>=0;j--){
					if(e[j]==0){
						if(e[j-1]!=1){
							e[j]=2;
							flg=1; break;
						}
					}
					else if(e[j]==1) j--;
				}
				if(flg==0){
					for(j=0;j<n;j++){
						if(e[j]==0){
							e[j]=2;
							break;
						}
					}
				}
			}
			else if(s=='C'){
				flg=0;
				if(i==0){
					e[n/2]=3;
				}
				else{
					for(j=1;j<n;j++){
						if(e[j-1]!=0&&e[j]==0) e[j]=3;flg=1; break;
					}
					if(flg==0){
						for(j=n-1;j>=0;j--){
							if(e[j]==0) e[j]=3;break;
						}
					}
				}
			}
			else if(s=='D'){
				flg=0;d=0; w=0;q=0;
				if(i==0){
					e[0]=4;
				}
				else {
					for(j=0;j<n;j++){
						if(e[j]==0) d++; flg=1;
						if(flg==1&&e[j]!=0){
							if(w<d){
								w=d;
								p=q;
								k=j-1;
							}
							d=0;
						}
						if(e[j]!=0) q=j;
					}
					e[p+w/2]=4;
				}
			}
		}
		for(i=0;i<n;i++){
			if(e[i]==0) printf("#");
			if(e[i]==1) printf("A");
			if(e[i]==2) printf("B");
			if(e[i]==3) printf("C");
			if(e[i]==4) printf("D");
		}
		printf("\n");
	}
	return 0;
}

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;}
    }
    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(!i){
	  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] && j < n-1){
	      chair[j+1] = 3;
	      break;
	    }
	    if(chair[j] && !chair[j-1] && j){
	      chair[j-1] = 3;
	      break;
	    }
	  }
	}
      }
      if(row[i] == 4){
	if(!i){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;
	      }
	    }
	  }
	}
      }
    }
    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;
}