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>
#include<vector>

using namespace std;

class A {
public:
	vector<char> set_chair(vector<char> chair) {
		for (int i = 1; i < chair.size()-1; i++) {
			if (chair[i] == '#') {
				chair[i] = 'A';
				return chair;
			}
		}
	}
};

class B {
public:
	vector<char> set_chair(vector<char> chair) {
		for (int i = chair.size()-2; i >= 1; i--) {
			if (chair[i] == '#' && chair[i + 1] != 'A' && chair[i - 1] != 'A') {
				chair[i] = 'B';
				return chair;
			}
		}
		for (int i = 1; i < chair.size()-1; i++) {
			if (chair[i] == '#') {
				chair[i] = 'B';
				return chair;
			}
		}
	}
};

class C {
public:
	vector<char> set_chair(vector<char> chair) {
		for (int i = 1; i < chair.size()-1; i++) {
			if (chair[i] != '#') {
				if (chair[i + 1] == '#' && i+1 != chair.size()-1) {
					chair[i + 1] = 'C';
					return chair;
				}else if (chair[i - 1] == '#' && i-1 != 0) {
					chair[i - 1] = 'C';
					return chair;
				}
			}
		}

		if (chair.size() % 2 == 1)chair[(chair.size() + 1) / 2 + 1] = 'C';
		else chair[chair.size() / 2 + 1 + 1] = 'C';
		return chair;

	}
};

class D {
public:
	vector<char> set_chair(vector<char> chair) {
		vector<int> numOfMostDistance(chair.size());
		for (int i = 1; i < chair.size() - 1; i++) {
			int rightMax = 0;
			int leftMax =  0;
			for (int j = 0; chair[i + j] == '#' && j+i != chair.size()-1; rightMax++,j++);
			for (int j = 0; chair[i - j] == '#' && i-j != 0; leftMax++,j++);
			numOfMostDistance[i] = min(rightMax, leftMax);
		}

		bool chairIsEmpty = true;
		for (int i = 1; i < chair.size() - 1; i++) {
			if (chair[i] != '#')chairIsEmpty = false;
		}

		if (chairIsEmpty)chair[1] = 'D';
		else chair[max_element(numOfMostDistance.begin(), numOfMostDistance.end()) - numOfMostDistance.begin()] = 'D';
		return chair;
	}
};


int main() {
	int n, m;
	while (cin >> n >> m, n, m) {
		vector<char> chair(n + 2,'#');
		for (int i = 0; i < m; i++) {
			char a; cin >> a;
			if (a == 'A') {
				A tmp;
				chair = tmp.set_chair(chair);
			}
			if (a == 'B') {
				B tmp;
				chair = tmp.set_chair(chair);
			}
			if (a == 'C') {
				C tmp;
				chair = tmp.set_chair(chair);
			}
			if (a == 'D') {
				D tmp;
				chair = tmp.set_chair(chair);
			}
		}
		for (int i = 1; i < chair.size() - 1; i++)cout << chair[i];
		cout << endl;
	}
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
vector<int> data;
int n,N;

int getLeft(){
	rep(i,n)if(data[i] == 0)return i;
	return -1;
}

int B(){
	for(int i=n-1;i>=0;i--){
		if(data[i] == 0 && data[i+1] != 1 && data[i-1] != 1)return i;
	}
	return getLeft();
}
int C(){
	bool f = true;
	rep(i,N)f &=(data[i+1]==0);
	if(f)return (N&1?n/2:N/2+1);
	
	rep(i,n){
		if(data[i] && data[i] != -1){
			if(data[i+1]==0)return i+1;
			else if(data[i-1]==0)return i-1;
		}
	}
}

int most(int pos,int a){
	int i = 0;
	while(!data[pos+i*a]){
		i++;
		if(data[pos+i*a] == -1)return 999999;
	}
	return i;
}

int D(){
	int good , val = -1;
	rep(i,n){
		if(data[i] == 0){
			int t = min( most(i,-1) , most(i,1) );
			if(t > val){
				val = t;
				good = i;
			}
		}
	}
	return good;
}
int main(){
	string tab = "#ABCD";
	int m;
	while(cin >> n >> m , n ){
		N = n ; n++;
		data.resize(N+2);
		fill(data.begin(),data.end(),0);
		data[0] = -1 , data[n] = -1;
		
		rep(i,m){
			char c; cin >> c;
			switch(c){
			case 'A':
				data[getLeft()] = 1;
				break;
			case 'B':
				data[B()] = 2;
				break;
			case 'C':
				data[C()] = 3;
				break;
			case 'D':
				data[D()] = 4;
				break;
			}
		}
		rep(i,N){
			int t = data[i+1];
			cout << tab[t];
		}
		cout << endl;
	}
}

C++: #include<iostream>
using namespace std;
#define rep(i,s,e) for ( int i = s; i <= e; i++ )
#define MAX 10000
char C[MAX+2];
int n, m;

void setLeft(char ch){
    for ( int i = 1; ; i++ ){
	if ( C[i] == '#' ) { C[i] = ch; return; }
    }
}

void setB(){
    for ( int i = n; i >= 1; i-- ){
	if ( C[i] == '#' && C[i-1] != 'A' && C[i+1] != 'A' ){
	    C[i] = 'B'; return;
	}
    }
    setLeft('B');
}

bool isEmpty(){
    rep(i,1,n) if ( C[i] != '#' ) return false;
    return true;
}

void setC(){
    if ( isEmpty() ){ C[n/2+1] = 'C'; return; }
    rep(i,1,n){
	if ( C[i] == '#' ) continue;
	if ( C[i+1] == '#' ) { C[i+1] = 'C'; return; }
	if ( C[i-1] == '#' ) { C[i-1] = 'C'; return; }
    }
}

int getDist( int p ){
    int l = 0, r = 0;
    while( C[p+l-1] == '#' ) l--;
    if ( C[p+l-1] == 'X' ) l = -1000000;
    while( C[p+r+1] == '#' ) r++;
    if ( C[p+r+1] == 'X' ) r = 1000000;
    return min( (-1)*l, r);
}
void setD(){
    if ( isEmpty() ){ C[1] = 'D'; return; }
    int d, mi, md = -1;
    rep(i,1,n){
	if ( C[i] != '#' ) continue;
	d = getDist(i);
	if ( d > md ) {
	    md = d; mi = i;
	}
    }
    C[mi] = 'D';
}

main(){
    char ch;
    while( cin >> n >> m && n && m ){
      assert( n <= 100 );
	rep(i,0,n+1) C[i] = '#';
	C[0] = C[n+1] = 'X';
	rep(i,1,m){ 
	    cin >> ch;
	    if ( ch == 'A' ) setLeft('A');
	    else if ( ch == 'B' ) setB();
	    else if ( ch == 'C' ) setC();
	    else if ( ch == 'D' ) setD();
	}
	rep(i,1,n) cout << C[i];
	cout << endl;
    }
}