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<iostream>
using namespace std;
#define REP(i,b,n) for(int i=b;i<n;i++)
#define rep(i,n)   REP(i,0,n)

int findlm(int n,char *in){
  rep(i,n)if(in[i] == 0)return i;
}

int findNotA(int n,char *in){
  for(int i=n-1;i>=0;i--){
    if (in[i] == 0 && (i==0||in[i-1]!= 'A') && (i == n-1 || in[i+1] != 'A'))return i;
  }
  return -1;
}

int nextP(int n,char *in){
  rep(i,n){
    if (in[i] != 0){
      if (i < n-1 && in[i+1] == 0)return i+1;
      if (i >0 && in[i-1] == 0)return i-1;
    }
  }
  return -1;
}

int notNextP(int n,char *in){
  int ret = -1,maxdist=0;
  rep(i,n){
    if( in[i] != 0)continue;
    int tmpdist = 0;
    for(int j=i-1;j>=0;j--){
      if (in[j] != 0){tmpdist=i-j;break;}
    }
    for(int j=i+1;j<n;j++){
      if (in[j] != 0){tmpdist=min(tmpdist,j-i);break;}
    }
    if (tmpdist > maxdist)ret=i,maxdist = tmpdist;
  }
  return ret;
}

void put(int n,char *in,char inp){
  int index = -1;
  if (inp == 'A'){
    index = findlm(n,in);
  }else if (inp == 'B'){
    index = findNotA(n,in);
    if (index == -1)index = findlm(n,in);
  }else if (inp == 'C'){
    index = nextP(n,in);
    if (index == -1)index =n/2;
  }else if (inp == 'D'){
    index = notNextP(n,in);
    if (index == -1)index = 0;
  }
  in[index] = inp;
}


main(){
  char op[255];
  op['A']='A';  op['B']='B';  op['C']='C';  op['D']='D';
  op[0]='#';

  int n,m;
  char in[100];
  while(cin>>n>>m && n){
    rep(i,n)in[i] = 0;
    rep(i,m){
      char inp;
      cin>>inp;
      put(n,in,inp);
    }
    rep(i,n)cout << op[in[i]] ;
    cout << endl;
  }


}

C++: #include <iostream>
#include <string>
#include <vector>
using namespace std;

const int N_MAX = 100;

void sitA( char* seat, int n )
{
	int i = 0;
	while( seat[ i ] != '#' ){
		++i;
	}
	seat[ i ] = 'A';
}

void sitB( char* seat, int n )
{
	int i = n - 1;
	while( i > 0 ){
		if( seat[ i ] == '#' ){
			if( seat[ i - 1 ] != 'A' && seat[ i + 1 ] != 'A' ){
				seat[ i ] = 'B';
				return;
			}
		}
		--i;
	}
	while( seat[ i ] != '#' ){
		++i;
	}
	seat[ i ] = 'B';
}

void sitC( char* seat, int n )
{
	int i = 0;
	while( i < n ){
		if( seat[ i ] != '#' ){
			if( seat[ i + 1 ] =='#' ){
				seat[ i + 1 ] = 'C';
				return;
			}
			if( i >= 0 && seat[ i - 1 ] == '#' ){
				seat[ i - 1 ] = 'C';
				return;
			}
		}
		++i;
	}
	seat[ n / 2 ] = 'C';
}

void sitD( char* seat, int n )
{
	int est[ N_MAX ];
	int s, e;
	vector< int > seated;
	for( int i = 0; i < n; ++i ){
		if( seat[ i ] != '#' ){
			seated.push_back( i );
		}
	}
	if( seated.size() == 0 ){
		seat[ 0 ] = 'D';
		return;
	}
	e = seated[ 0 ];
	for( int i = e; i >= 0; --i ){
		est[ i ] = e - i;
	}
	for( int i = 1; i < seated.size(); ++i ){
		s = e;
		e = seated[ i ];
		for( int j = 0; j <= ( e - s ) / 2; ++j ){
			est[ s + j ] = est[ e - j ] = j;
		}
	}
	for( int i = e; i < n; ++i  ){
		est[ i ] = i - e;
	}
	int estMax = 0, estIdx = 0;
	for( int i = 0; i < n; ++i ){
		if( est[ i ] > estMax ){
			estMax = est[ i ];
			estIdx = i;
		}
	}
	seat[ estIdx ] = 'D'; 
}

void sit( char a, char* seat, int n )
{
	switch( a ){
		case 'A': sitA( seat, n ); break;
		case 'B': sitB( seat, n ); break;
		case 'C': sitC( seat, n ); break;
		case 'D': sitD( seat, n ); break;
	}
}

int main()
{
	int n, m;
	char seat[ N_MAX + 1 ];
	while( cin >> n >> m && ( n || m ) ){
		for( int i = 0; i < n; ++i ){
			seat[ i ] = '#';
		}
		seat[ n ] = '\0';

		for( int i = 0; i < m; ++i ){
			char a;
			cin >> a;
			sit( a, seat, n );
		}
		cout << seat << endl;
	}
	return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
	int n, m;
	char str[10001];
	
	while (scanf("%d %d", &n, &m) && n){
		memset(str, '#', sizeof(str));
		vector<int> used;
		str[n] = 0;
		for (int i = 0; i < m; i++){
			char p;
			scanf(" %c", &p);
			
			if (p == 'A'){
				for (int pos = 0; pos < n; pos++){
					if (str[pos] == '#'){
						str[pos] = 'A';
						used.push_back(pos);
						break;
					}
				}
			}
			if (p == 'B'){
				bool seated = false;
				for (int pos = n - 1; pos; pos--){
					if (str[pos] == '#' && (pos == 0 || str[pos - 1] != 'A')){
						str[pos] = 'B';
						used.push_back(pos);
						seated = true; break;
					}
				}
				if (!seated)
					for (int pos = 0; pos < n; pos++){
					if (str[pos] == '#'){
						str[pos] = 'B'; used.push_back(pos);
						break;
					}
				}
			}
			if (p == 'C'){
				if (i == 0){
					if (n & 1){
						str[(n + 1) / 2 - 1] = 'C';
						used.push_back((n + 1) / 2 - 1);
					}
					else {
						str[n / 2] = 'C';
						used.push_back(n / 2);
					}
				}
				else {
					for (int pos = 0; pos < n; pos++){
						if (str[pos] != '#'){
							if (pos + 1 < n && str[pos + 1] == '#'){
								str[pos + 1] = 'C';
								used.push_back(pos + 1);
								break;
							}
							if (pos - 1 >= 0 && str[pos - 1] == '#'){
								str[pos - 1] = 'C';
								used.push_back(pos - 1);
								break;
							}
						}
					}
				}
			}
			if (p == 'D'){
				if (i == 0){
					str[0] = 'D';
					used.push_back(0);
				}
				else {
					sort(used.begin(), used.end());
					int minDist = 999999, minPos = -1;
					for (int pos = 0; pos < n; pos++){
						if (str[pos] == '#'){
							int a = upper_bound(used.begin(), used.end(), pos) - used.begin() - 1;
							int b = upper_bound(used.begin(), used.end(), pos) - used.begin();
							int tDist = 0;
							if (a != -1) tDist = max(tDist, pos - used[a]);
							if (b != used.size()) tDist = max(tDist, used[b] - pos);
							if (tDist < minDist){
								minDist = tDist;
								minPos = pos;
							}
						}
					}
					str[minPos] = 'D';
					used.push_back(minPos);
				}
			}
		}
		printf("%s\n", str);
	}
}