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>
#include <vector>
#include <climits>
using namespace std;

int n, m;
string seat;

void sit(char a) {
    switch (a) {
        case 'A': {
            for (int i=1; i<=n; ++i) {
                if (seat[i] == '#') {
                    seat[i] = 'A'; return ;
                }
            }
            break;
        }
        case 'B': {
            for (int i=n; i>=1; --i) {
                if (seat[i] == '#' && seat[i-1] != 'A' && seat[i+1] != 'A') {
                    seat[i] = 'B'; return ;
                }
            }
            for (int i=1; i<=n; ++i) {
                if (seat[i] == '#') {
                    seat[i] = 'B'; return ;
                }
            }
            break;
        }
        case 'C': {
            bool empty = true;
            for (int i=1; i<=n; ++i) {
                if (seat[i] != '#') {
                    empty = false; break;
                }
            }
            if (empty) {
                seat[(n+2)/2] = 'C'; return ;
            }
            for (int i=1; i<=n; ++i) {
                if (seat[i] != '#') {
                    if (seat[i+1] == '#') {
                        seat[i+1] = 'C'; return ;
                    } else if (seat[i-1] == '#') {
                        seat[i-1] = 'C'; return ;
                    }
                }
            }
            break;
        }
        case 'D': {
            int k, mx = -1;
            for (int i=1; i<=n; ++i) {
                int l = 0;
                while (0 < i-l && seat[i-l] == '#') ++l;
                int r = 0;
                while (i+r < n+1 && seat[i+r] == '#') ++r;

                if (i+r == n+1) r = INT_MAX;
                if (i-l == 0) l = INT_MAX;
                int dist = min(l, r);

                if (mx < dist) {
                    k = i;
                    mx = dist;
                }
            }
            seat[k] = 'D';
            break;
        }
    }
}

int main() {
    while (cin >> n >> m, (n || m)) {
        seat = "_";
        for (int i=0; i<n; ++i) {
            seat += "#";
        }
        seat += "_";
        char a;
        for (int i=0; i<m; ++i) {
            cin >> a;
            sit(a);
        }
        cout << seat.substr(1,n) << endl;
    }
    return 0;
}