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>
#include <algorithm>

using namespace std;

const int INF = 1000000000;

int n, m;
char chairs[100];
int dist[100];
int p;

bool notA(int i){
   if(i == n-1){
      return chairs[i-1] != 'A';
   }else if(i == 0){
      return chairs[i+1] != 'A';
   }else{
      return chairs[i-1] != 'A' && chairs[i+1] != 'A';
   }
}

int checkC(int idx){
   if(idx == n-1){
      if(chairs[idx-1] == '#') return -1;
      return 0;
   }else if(idx == 0){
      if(chairs[idx+1] == '#') return 1;
      return 0;
   }else{
      if(chairs[idx+1] == '#') return 1;
      if(chairs[idx-1] == '#') return -1;
      return 0;
   }
}

void setD(int idx){
   for(int i=0;idx-i>=0;i++){
      dist[idx-i] = min(dist[idx-i], i);
   }
   for(int i=0;idx+i<n;i++){
      dist[idx+i] = min(dist[idx+i], i);
   }
}

void setChar(char c){
   if(n == 1){
      chairs[0] = c;
      return;
   }
   switch(c){
      case 'A':
         for(int i=0;i<n;i++){
            if(chairs[i] == '#'){
               chairs[i] = 'A';
               setD(i);
               return;
            }
         }
         break;
      case 'B':
         for(int i=n-1;i>=0;i--){
            if(notA(i)){
               chairs[i] = 'B';
               setD(i);
               return;
            }
         }
         for(int i=0;i<n;i++){
            if(chairs[i] == '#') chairs[i] = 'B';
            cout << i << endl;
            setD(i);
            return;
         }
         break;
      case 'C':
         if(p == 0){
            if(n&1){
               chairs[(n+1)/2-1] = 'C';
               setD((n+1)/2-1);
            }else{
               chairs[n/2] = 'C';
               setD(n/2);
            }
            return;
         }
         for(int i=0;i<n;i++){
            if(chairs[i] != '#'){
               int tmp = checkC(i);
               if(tmp == 1){
                  chairs[i+1] = 'C';
                  setD(i+1);
                  return;
               }else if(tmp == -1){
                  chairs[i-1] = 'C';
                  setD(i-1);
                  return;
               }
            }
         }
         break;
      case 'D':
         int idx = 0;
         int valm = 0;
         for(int i=0;i<n;i++){
            if(dist[i] > valm){
               valm = dist[i];
               idx = i;
            }
         }
         chairs[idx] = 'D';
         setD(idx);
         break;
   }
}

main(){
   while(cin >> n >> m && (n || m)){
      fill(dist, dist+n, INF);
      fill(chairs, chairs+n, '#');
      char c;
      for(p=0;p<m;p++){
         cin >> c;
         setChar(c);
      }
      for(int i=0;i<n;i++){
         cout << chairs[i];
      }
      cout << endl;
   }
   return 0;
}

C++: #include <algorithm>
#include <cctype>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <string>
#include <vector>

using namespace std;

#define reps(i,f,n) for(int i=f; i<int(n); ++i)
#define rep(i,n) reps(i,0,n)

typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int, int> pii;

const int INF = 1001001001;

void sit(char c, char chair[], int n)
{
	if(c == 'A'){
		rep(i, n){
			if(chair[i] == '#'){
				chair[i] = 'A';
				return;
			}
		}
	}
	else if(c == 'B'){
		int pos = -1;
		for(int i=n-1; i>=0; --i){
			if(chair[i] == '#'){
				pos = i;
				if(i!=0 && chair[i-1]=='A')
					continue;
				if(i!=n-1 && chair[i+1]=='A')
					continue;
				chair[i] = 'B';
				return;
			}
		}
		chair[pos] = 'B';
	}
	else if(c == 'C'){
		rep(i, n){
			if(chair[i] != '#'){
				if(i!=n-1 && chair[i+1]=='#'){
					chair[i+1] = 'C';
					return;
				}
				if(i!=0 && chair[i-1]=='#'){
					chair[i-1] = 'C';
					return;
				}
			}
		}
		chair[n/2] = 'C';
	}
	else{
		int B = 0;
		int Bid;
		rep(i, n){
			int dist = INF;
			rep(j, n){
				if(chair[j] != '#')
					dist = min(dist, abs(i-j));
			}
			if(dist > B){
				B = dist;
				Bid = i;
			}
		}
		chair[Bid] = 'D';
	}
}

int main()
{
	int n, m;
	while(scanf("%d%d", &n, &m), n){
		char chair[100];
		fill(chair, chair+n, '#');
		rep(i, m){
			char c;
			scanf(" %c", &c);
			sit(c, chair, n);
		}
		rep(i, n)
			putchar(chair[i]);
		puts("");
	}
	return 0;
}