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;
#define MAX 100
int main(){
	int seat,people;
	while(1){
		cin>>seat>>people;
		if(seat+people==0 )break;
		string str[101],x;
		for( int i=0 ; i<=MAX ; i++ )
			str[i]="#";
		for( int i=0 ; i<people ; i++ ){
			cin>>x;
			if(x=="A"){
				for( int j=0 ; j<seat ; j++ ){
					if(str[j]=="#"){
						str[j]="A";
						break;
					}
				}
			}

			else if(x=="B"){
				int all_b_sign=0;
				for(int j=seat-1 ; j>=0 ; j-- ){
					if(str[j]=="#"){
						int B_sign=0;
						if(j==0)
							B_sign=1;
						else if(j==seat-1)
							if(str[j-1]!="A")
								B_sign=1;
						else if(str[j+1]!="A"&&str[j-1]!="A")
							B_sign=1;
						if(B_sign==1){
							str[j]="B";
							all_b_sign=1;
							break;
						}
					}
				}
				if(all_b_sign==0){
					for(int j=0 ; j<seat ; j++ ){
						if(str[j]=="#"){
							str[j]="B";
							break;
						}
					}
				}
			}

			else if(x=="C"){
				int c_sign=0;
				for(int j=0 ; j<seat ; j++ ){
					if(str[j]!="#"){
						if(j==seat-1){
							if(str[j-1]=="#"){
								str[j-1]="C";
								c_sign=1;
								break;
							}
						}
						else if(j==0){
							if(str[j+1]=="#"){
								str[j+1]="C";
								c_sign=1;
								break;
							}
						}
						else if(str[j+1]=="#"){
							str[j+1]="C";
							c_sign=1;
							break;
						}
						else if(str[j-1]=="#"){
							str[j-1]="C";
							c_sign=1;
							break;
						}

					}
				}
				if(c_sign==0){
					if(seat%2)
						str[(seat+1)/2]="C";
					else
						str[seat/(2+1)]="C";
				}
			}

			else if(x=="D"){
				if(i==0){
					str[0]="D";
					continue;
				}
				int location,max=0;
				for( int j=seat-1 ; j>=0 ; j-- ){
					int l=j;
					while(l>0){
						if(str[l]!="#"){
							if(max<=j-l){
								max=j-l;
								location=j;
							}
							break;
						}
						l--;
					}
					l=j;
					while(l<seat){
						if(str[l]!="#"){
							if(max<=l-j){
								max=l-j;
								location=j;
							}
							break;
						}
						l++;
					}
				}
				str[location]="D";
			}

		}
		for(int i=0 ; i<seat ; i++ )
			cout<<str[i];
		cout<<endl;
	}
}

C++: #include<iostream>
#include<algorithm>
const int infty = 1<<28;
using namespace std;
int main()
{
  while(true){
    int n,m;
    cin >> n >> m;
    if( n == 0 && m == 0 ) break;
    char C[n+2];
    fill(C,C+n+2,'#');
    for(int p = 0; p < m; ++p){
      char c;
      cin >> c;
      if(c=='A'){
	for(int i = 1; i < 1+n; ++i){
	  if( C[i] == '#' ){
	    C[i] = 'A';
	    break;
	  }
	}
      }else if(c=='B'){
	bool sc=false;
	for(int i = n; i >= 1; --i){
	  if( C[i] == '#' && C[i-1] != 'A' && C[i+1] != 'A' ){
	    C[i] = 'B';
	    sc=true;
	    break;
	  }
	}
	if(!sc){
	  for(int i = 1; i < n+1; ++i){
	    if( C[i] == '#' ){
	      C[i] = 'B';
	      break;
	    }
	  }
	}
      }else if(c=='C'){
	bool sc = false;
	for(int i = 1; i < 1+n; ++i){
	  if( C[i-1] != '#' ){
	    if( C[i] != '#' ) continue;
	    else{
	      C[i] = 'C';
	      sc = true;
	      break;
	    }
	  }
	}
	if( !sc ){
	  if( n % 2 == 0 ){
	    C[ n/2 + 1 ] = 'C';
	  }else{
	    C[ (n+1)/2 ] = 'C';
	  }
	}
      }else if(c=='D'){
	int max_dist = 0;
	int nearest_dist[n+2];
	fill(nearest_dist,nearest_dist+n+2,infty);
	bool empty=true;
	for(int i = 1; i < 1+n; ++i){
	  if( C[i] != '#' ) empty=false;
	}
	if(empty){C[1]='D';continue;}
	
	for(int i = 1; i < 1+n; ++i){
	  if( C[i] != '#' ) continue;
	  int min_right = infty, min_left = infty;
	  for(int j = i - 1; j >= 0; --j){
	    if( C[j] != '#'){
	      min_left = min( min_left, i - j );
	      break;
	    }
	  }
	  for(int j = i + 1; j < 2+n; ++j){
	    if( C[j] != '#'){
	      min_right = min( min_right,  j - i );
	      break;
	    }
	  }

	  nearest_dist[i] = min( min_right, min_left );
	  max_dist = max( max_dist, nearest_dist[i] );
	}
	for(int i = 1; i < i+n; ++i){
	  //cout << nearest_dist[i] << ' ' << max_dist << endl;
	  if( nearest_dist[i] == max_dist ){
	    C[i] = 'D';
	    break;
	  }
	}
      }
    }
    for(int i = 1; i < 1+n; ++i){
      cout << C[i];
    }
    cout << endl;
  }
  return 0;
}