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<cstring>
#include<algorithm>
#include<vector>
#include<map>

using namespace std;

int N, M;
char ChairsPerson[101];
int Offset[101];


void solveA() {

	for (int i = 1; i <= N; ++i) {
		if (ChairsPerson[i]=='#') {
			ChairsPerson[i] = 'A';
			return;
		}
	}
}

void solveB() {

	for (int i = N; i >= 1; --i) {
		if (ChairsPerson[i] != '#') {
			continue;
		}
		if (i > 1 && ChairsPerson[i - 1] == 'A') {
			i -= 2;
			continue;
		}
		if (i < N  && ChairsPerson[i + 1] == 'A') {
			continue;
		}
		ChairsPerson[i] = 'B';
		return;
	}

	for (int i = 1; i <= N; ++i) {
		if (ChairsPerson[i] == '#') {
			ChairsPerson[i] = 'B';
			return;
		}
	}
}

void solveC(int id) {

	if (id == 0) {
		//firstperson
		ChairsPerson[N / 2 + 1] = 'C';
		return;
	}

	for (int i = 1; i <= N; ++i) {
		if (ChairsPerson[i] == '#') {
			continue;
		}
		if (i < N && ChairsPerson[i + 1] == '#') {
			ChairsPerson[i + 1] = 'C';
			return;
		}
		if (i > 1 && ChairsPerson[i - 1] == '#') {
			ChairsPerson[i - 1] = 'C';
			return;
		}
	}	
}

void solveD() {
	int pos, pos1, pos2, at;

	for (pos1 = 1; pos1 <= N; ++pos1) {
		if (ChairsPerson[pos1] == '#') {
			continue;
		}
		for (pos2 = 1; pos2 <= N; ++pos2) {
			Offset[pos2] = min(Offset[pos2], abs(pos1 - pos2));
		}
	}

	pos = 0;
	at = 1;
	for (pos1 = 1; pos1 <= N; ++pos1) {
		//cout << "pos1:" << pos1 << "  " << Offset[pos1] << endl;
		if (Offset[pos1] > pos) {
			pos = Offset[pos1];
			at = pos1;
		}
	}
	ChairsPerson[at] = 'D';
}

int main() {
	char person;

	while (cin >> N >> M, N) {
		fill(ChairsPerson + 1, ChairsPerson + N + 1, '#');
		fill(Offset + 1, Offset + N + 1, 1000);

		for (int i = 0; i < M; ++i) {
			cin >> person;
			switch (person)
			{
			case 'A':solveA(); break;
			case 'B':solveB(); break;
			case 'C':solveC(i); break;
			case 'D':solveD(); break;
			default:break;
			}
		}
		for (int i = 1; i <= N; ++i) {
			cout << ChairsPerson[i];
		}
		cout << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <math.h>
#include <vector>
#include <set>

using namespace std;
typedef long long LL;
static const double EPS = 1e-9;

#define FOR(i,k,n) for (int i=(k); i<(int)(n); ++i)
#define REP(i,n) FOR(i,0,n)
#define P(x) cout << (x) << endl

int main(void){  
  int n,m;
  while(cin>>n>>m){
    if(!n) break;
    int ch[102] = {0};
    char a;
    REP(h,m){
      cin >> a;
      if(a == 'A'){
        FOR(i,1,n+1){
          if(ch[i] == 0){
            ch[i] = 'A';
            break;
          }
        }
      }else if(a == 'B'){
        bool f=false;
        for(int i=n; i>0; i--){
          if(ch[i]==0 && ch[i+1] != 'A' && ch[i-1] != 'A'){
            ch[i] = 'B';
            f = true;
            break;
          }
        }
        if(!f){
          FOR(i,1,n+1){
            if(ch[i] == 0){
              ch[i] == 'B';
              break;
            }
          }
        }
      }else if(a == 'C'){
        int c=-1;
        FOR(i,1,n+1){
          if(ch[i] != 0){
            c = i;
            break;
          }
        }
        if(c==-1){
          ch[(n%2)?(n+1)/2:n/2+1] = 'C';
        }else{
          while(c<n+1){
            if(ch[c+1]==0&&c<n){
              ch[c+1] = 'C';
              break;
            }else if(ch[c-1]==0&&c>1){
              ch[c-1] = 'C';
              break;
            }
            c++;
          }
        }
      }else if(a == 'D'){
        int maxi,max=0;
        FOR(i,1,n+1){
          if(ch[i] == 0){
            int l=100,r=100,mi;
            for(int j=i-1; j>0; j--)
              if(ch[j] != 0){
                l = i-j;
                break;
              }
            for(int j=i+1; j<n+1; j++)
              if(ch[j] != 0){
                r = j-i;
                break;
              }
            mi = min(l,r);
            if(mi>max){
              maxi = i;
              max = mi;
            }
          }
        }
        ch[maxi] = 'D';
      }
    }
    for(int i=1; i<n+1; i++){
      if(ch[i] == 0) cout <<"#";
      else printf("%c",ch[i]);
    }
    cout << endl;
  }
  return 0;
}

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

using namespace std;

int main()
{
	for(int n,m;cin>>n>>m,n|m;){
		string s(n,'#');
		while(m--){
			char p; cin>>p;
			if(p=='A'){
				for(int i=0;i<n;i++)
					if(s[i]=='#'){
						s[i]='A';
						break;
					}
			}
			if(p=='B'){
				bool flg=false;
				for(int i=n-1;i>=0;i--){
					if(s[i]=='#' && (i==n-1 || s[i+1]!='A') && (i==0 || s[i-1]!='A')){
						s[i]='B';
						flg=true;
						break;
					}
				}
				if(!flg){
					for(int i=0;i<n;i++)
						if(s[i]=='#'){
							s[i]='A';
							break;
						}
				}
			}
			if(p=='C'){
				if(count(s.begin(),s.end(),'#')==n)
					s[s.size()/2]='C';
				else{
					for(int i=0;i<n;i++){
						if(s[i]=='#')
							continue;
						if(i<n-1 && s[i+1]=='#'){
							s[i+1]='C';
							break;
						}
						if(i>0 && s[i-1]=='#'){
							s[i-1]='C';
							break;
						}
					}
				}
			}
			if(p=='D'){
				if(count(s.begin(),s.end(),'#')==n)
					s[0]='D';
				else{
					int mi=-1,md;
					for(int i=0;i<n;i++){
						if(s[i]!='#')
							continue;
						int d=n;
						for(int j=i-1;j>=0;j--)
							if(s[j]!='#'){
								d=min(d,i-j);
								break;
							}
						for(int j=i+1;j<n;j++)
							if(s[j]!='#'){
								d=min(d,j-i);
								break;
							}
						if(mi==-1 || d<md){
							mi=i;
							md=d;
						}
					}
					s[mi]='D';
				}
			}
		}
		cout<<s<<endl;
	}
	
	return 0;
}