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 <string>
using namespace std;

char map[101];
int n,m;

void sheet(char hit[2]){
	int a=0;
	bool ch=false;
	if(hit[0]=='A'){
		while(1){
			if(map[a]=='#'){
				map[a]='A';
				break;
			}
			a++;
		}
	}
	if(hit[0]=='B'){
		for(a=n-1;a>=0;a--){
			if(map[a]=='#' && map[a-1]!='A' && map[a+1]!='A'){
				map[a]='B';
				ch=true;
				break;
			}
		}
		if(!ch){
			while(1){
				if(map[a]=='#'){
					map[a]='B';
					break;
				}
				a++;
			}
		}
	}
	if(hit[0]=='C'){
		for(a=0;a<n;a++){
			if(map[a]=='#' &&((map[a-1]!='#' && a>0)||(map[a+1]!='#' && a<n-1))){
				map[a]='C';
				ch=true;
				break;
			}
		}
		if(!ch){
			if(n%2==0)map[n/2]='C';
			if(n%2==1)map[(n+1)/2-1]='C';
		}
	}
	if(hit[0]=='D'){
		int maxIndex = -1, dist = -1,i,j;
		for (i = 0; i < n; ++i) {
			if(map[i] != '#')continue;
			int right = 0;
			if(i == n-1)right = n;
			for(j = i+1; j < n; ++j) {
				if (map[j] == '#')++right;
				else break;
			}
			int left = 0;
			if (i == 0)left = n;
			for (j = i-1; j >= 0; --j) {
				if (map[j] == '#')++left;
				else break;
			}
			if (dist < min(right, left)) {
				maxIndex = i;
			    dist = min(right, left);
			}
		}
		map[maxIndex] = 'D';
    }
}

int main(void){
	int a;
	while(1){
		for(a=0;a<=100;a++){
			map[a]='#';
		}
		cin >> n >> m;
		if(n==0 && m==0)break;
		for(a=0;a<m;a++){
			char hito[2];
			cin >> hito;
			sheet(hito);
		}
		for(a=0;a<n;a++)cout << map[a];
		cout << endl;
	}
	return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <cstring>
#include <sstream>
#include <cassert>
#include <list>
using namespace std;
static const double EPS = 1e-10;
typedef long long ll;
typedef pair<int,int> PI;
typedef vector<int> vi;
#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define F first
#define S second

int dx[]={0,1,0,-1},dy[]={1,0,-1,0};

int seki[111];

main(){
  int m,n;
  while(cin>>n>>m,n|m){
    string seat;
    rep(i,n)seat+='#';

    rep(i,m){
      cout<<seat<<" "<<m-i<<endl;
      char c;
      int pos=0;
      cin>>c;
      switch(c){
      case 'A':
	pos=0;
	while(seat[pos]!='#')++pos;
	seat[pos]='A';
	break;
      case 'B':
	pos=n-1;
	while(true){
	  if(pos<0)break;
	  if(seat[pos]!='#'){
	    --pos;
	    continue;
	  }
	  if(pos && seat[pos-1]=='A'){
	    --pos;
	    continue;
	  }
	  if(pos+1<n && seat[pos+1]=='A'){
	    --pos;
	    continue;
	  }
	  seat[pos]='B';
	  break;
	}
	if(pos==-1){
	  pos=0;
	  while(seat[pos]!='#')++pos;
	  seat[pos]='B';
	}
	break;
      case 'C':
	if(i==0){
	  if(n%2)seat[(n+1)/2-1]='C';
	  else seat[n/2]='C';
	}else{
	  pos=0;
	  while(seat[pos]!='#')++pos;
	  if(pos)seat[pos]='C';
	  else{
	    pos=0;
	    while(seat[pos]=='#')++pos;
	    int t=pos-1;
	    while(pos<n && seat[pos]!='#')++pos;
	    if(pos==n)seat[t]='C';
	    else seat[pos]='C';
	  }
	}
	break;
      case 'D':
	if(i==0){
	  seat[0]='D';
	  continue;
	}
	memset(seki,0,sizeof(0));
	int maxt=0,maxpos=0;

	if(seat[0]=='#'){
	  seki[0]=1;
	  maxt=1;
	  maxpos=1;
	}
	pos=1;
	while(pos<n){
	  if(seat[pos]=='#')seki[pos]=seki[pos-1]+1;
	  else seki[pos]=0;
	  if(seki[pos]>maxt){
	    maxt=seki[pos];
	    maxpos=pos;
	  }
	  ++pos;
	}
	//rep(j,n)cout<<seki[j]<<" ";
	//cout<<endl;
	//cout<<maxpos<<" "<<maxt<<endl;
	if(maxpos==n-1)seat[n-1]='D';
	else if(maxpos==maxt-1)seat[0]='D';
	else if(maxt%2)seat[maxpos-maxt/2]='D';
	else seat[maxpos-maxt/2]='D';
	break;
      }
    }
    cout<<seat<<endl;
  }
}

C++: #include<iostream>
#include<algorithm>
using namespace std;
int main() {
	int n, m;
	while (cin >> m >> n&&n != 0 && m != 0) {
		char a[100];
		fill(a, a + m, '#');
		for (int z = 0; z < n; z++) {
			char b;
			cin >> b;
			switch (b) {
				case 'A':
					for (int i = 0; i < m; i++) {
						if (a[i] == '#') {
							a[i] = 'A';
							break;
						}
					}
					break;
				case 'B': 
					for (int i = m - 1; i >= 0; i--) {
						if ((i == (m - 1) && a[m - 2] != 'A'&&a[i] == '#') || (i == 0 && a[1] != 'A'&&a[i] == '#') || (a[i - 1] != 'A'&&a[i] == '#'&&a[i + 1] != 'A')) {
							a[i] = 'B';
							goto stop;
						}
					}
					for (int i = 0; i < m; i++) {
						if (a[i] == '#') {
							a[i] = 'B';
							break;
						}
					}
				stop:;
					break;
				case 'C':
					if (z == 0) {
						if (m % 2 == 1)
							a[(m + 1) / 2] = 'C';
						else
							a[m / 2 + 1] = 'C';
						goto st;
					}
					for (int i = 1; i < m; i++) {
						if (a[i - 1] != '#'&&a[i] == '#') {
							a[i] = 'C';
							goto st;
						}
					}
				st:;
					break;
				case 'D':
					int f = 0, g = 0, o = 0, p = 0;
					if (z == 0) {
						a[0] = 'D'; goto stp;
					}
					for (int i = m - 1; i >= 0; i--) {
						int h = 0, k = 10000;
						for (int j = i; j >= 0; j--) {
							if (a[j] != '#')
								break;
							else {
								o++;
								h++;
							}
						}
						for (int j = i; j < m; j++) {
							if (a[j] != '#')
								break;
							else {
								if (j +1!= m) k = 0;
								p++;
								k++;
							}
						}
						if (g <= min(h, k)) {
							f = i;
							g = min(h, k);
						}
					}
					a[f] = 'D';
				stp:;
					break;
			}
		}
		for (int i = 0; i < m; i++) {
			cout << a[i];
		}
		cout << endl;
	}
}