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 <algorithm>
 
using namespace std;

#define MAX 100

int n,m;
char chair[MAX+2];
  
void A()
{
    for (int i = 1; i <= n; i++) {
	if (chair[i] == '#') {
	    chair[i] = 'A';
	    return;
	}
    }
}
 
void B()
{
    int pos = -1;
    for (int i = 1; i <= n; i++) {
	if (chair[i] == 'A') {
	    pos = i;
	    break;
	}
    }
   
    if (pos == -1) {
	for (int i = n; i > 0; i--) {
	    if (chair[i] == '#') {
		chair[i] = 'B';
		return;
	    } 
	}
    } else {
	for (int i = n; i > 0; i--) {
	    if (chair[i] == '#' && chair[i-1] != 'A' && chair[i+1] != 'A') {
		chair[i] = 'B';
		return;
	    }
	}
  
	for (int i = 1; i <= n; i++) {
	    if (chair[i] == '#') {
		chair[i] = 'B';
		return;
	    }
	}
    }
}
  
void C()
{
    int cnt = count(chair, chair+n, '#');
  
    if (cnt == n) {
	if (n % 2) {
	    chair[(n+1)/2] = 'C';
	} else {
	    chair[n/2+1] = 'C';
	}
    } else {
	for (int i = 1; i <= n; i++) {
	    if (chair[i] != '#' && chair[i+1] == '#' && i != n) {
		chair[i+1] = 'C';
		return;
	    }
	    if (chair[i] != '#' && chair[i-1] == '#' && i != 1) {
		chair[i-1] = 'C';
		return;
	    }
	}
    }
}
  
void D()
{
    int Breadth = 0, pos;
    int cnt = count(chair, chair+n, '#');
 
    if (cnt == n) {
	chair[1] = 'D';
	return;
    } 
 
    for (int i = 1; i <= n; i++) {
	int dist = (1<<29);
	if (chair[i] != '#') continue;
	for (int j = 1; j <= n; j++) {
	    if (chair[j] != '#') {
		dist = min(dist, abs(j-i)+1);
	    }
	}
	if (Breadth < dist) {
	    Breadth = dist;
	    pos = i;
	} 
    }
  
    chair[pos] = 'D';
}
  
int main()
{
    while (cin >> n >> m, n | m) {
	char person;
  
	for (int i = 0; i <= n+1; i++) {
	    chair[i] = '#';
	}
  
	for (int i = 0; i < m; i++) {
	    cin >> person;
       
	    switch (person) {
                case 'A': 
                    A(); 
                    break;
                case 'B': 
                    B(); 
                    break;
                case 'C': 
                    C(); 
                    break;
                case 'D': 
                    D(); 
                    break;
	    }
	}
	for (int i = 1; i <= n; i++) {
	    cout << chair[i];
	}
	cout << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>

int main() {
	int n, m;
	const char output[] = { '#', 'A', 'B', 'C', 'D' };
	std::vector<int> seat;
	
	while (std::cin >> n >> m) {
		if (n == 0 && m == 0)
			break;

		seat.resize(n);

		for (int i = 0; i < m; ++i) {
			char who;
			std::cin >> who;

			if (who == 'A') {
				for (int j = 0; j < n; ++j) {
					if (seat[j] == 0) {
						seat[j] = 1;
						break;
					}
				}
			}
			else if (who == 'B') {
				int firstEmpty = 334;
				bool isRightA = false;

				for (int j = n - 1; j >= 0; --j) {
					if (seat[j] == 0) {
						firstEmpty = std::min(firstEmpty, j);
						if (!isRightA && (j == 0 || seat[j - 1] != 1)) {
							seat[j] = 2;
							break;
						}
					}
					isRightA = seat[j] == 1;
				}
			}
			else if (who == 'C') {
				for (int j = 0; j < n; ++j) {
					if (seat[j] != 0) {
						if (j != n - 1 && seat[j + 1] == 0) {
							seat[j + 1] = 3;
							break;
						}
						if (j != 0 && seat[j - 1] == 0) {
							seat[j - 1] = 3;
							break;
						}
					}
					if (j == n - 1)
						seat[n / 2] = 3;
				}
			}
			else if (who == 'D') {
				int maxDist = 0;
				int maxIdx = 0;

				int dist = 0;

				int fromTop = 0;
				int toLast = 0;

				for (int j = 0; j < n; ++j) {
					++dist;
					if (seat[0] != 0 && j == n - 1)
						toLast = dist;

					if (seat[j] != 0) {
						if (fromTop == 0)
							fromTop = j + 1;

						if (maxDist < dist / 2) {
							maxDist = dist / 2;
							maxIdx = j - dist;
						}
						dist = 0;
					}
				}

				if (seat[n - 1] == 0 && toLast > maxDist && fromTop < toLast)
					seat[n - 1] = 4;
				else if (seat[0] == 0 && fromTop > maxDist && fromTop > toLast)
					seat[0] = 4;
				else
					seat[maxIdx + maxDist] = 4;
			}
		}

		for (int i = 0; i < n; ++i)
			std::cout << output[seat[i]];
		std::cout << std::endl;
		seat.clear();
	}

	return 0;
}