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 <vector>
#include <queue>
#include <cmath>

using namespace std;

typedef pair<int, int> P;

int main() {
	int n, m;
	while (cin >> n >> m) {
		if (n == 0 && m == 0) {
			break;
		}
		vector<char> isu(n, '#');
		char c;
		for (int i = 0; i < m; i++) {
			cin >> c;
			if (c == 'A') {
				for (int j = 0; j < n; j++) {
					if (isu[j] == '#') {
						isu[j] = 'A';
						break;
					}
				}
			} else if (c == 'B') {
				bool hantei = true;
				for (int j = n-1; j >= 0; j--) {
					if (isu[j] == '#') {
						if (j != 0 && j != n-1) {
							if (isu[j-1] != 'A' && isu[j+1] != 'A') {
								isu[j] = 'B';
								hantei = false;
								break;
							}
						} else if (j == 0) {
							if (isu[j+1] != 'A') {
								isu[j] = 'B';
								hantei = false;
								break;
							}
						} else {
							if (isu[j-1] != 'A') {
								isu[j] = 'B';
								hantei = false;
								break;
							}
						}
					}
				}
				if (hantei) {
					for (int j = 0; j < n; j++) {
						if (isu[j] =='#') {
							isu[j] = 'B';
							break;
						}
					}
				}
			} else if (c == 'C') {
				bool hantei = true;
				for (int j = 0; j < n; j++) {
					if (isu[j] != '#') {
						if (j != n-1 && isu[j+1] == '#') {
							isu[j+1] = 'C';
							hantei = false;
							break;
						} else if (j != 0 && isu[j-1] == '#') {
							isu[j-1] = 'C';
							hantei = false;
							break;
						}
					}
				}
				if (hantei) {
					isu[n/2] = 'C';
				}
			} else {
				queue<P> q;
				for (int j = 0; j < n; j++) {
					if (isu[j] == '#') {
						int counter = 1000;
						int counter2 = 1000;
						int d;
						for (int k = j-1; k >= 0; k--) {
							if (isu[k] != '#') {
								counter = j-k;
								break;
							}
						}
						for (int k = j+1; k < n; k++) {
							if (isu[k] != '#') {
								counter2 = k-j;
								break;
							}
						}
						d = min(counter, counter2);
						if (d == 1000) {
							break;
						}
						if (q.empty()) {
							P p;
							p.first = d;
							p.second = j;
							q.push(p);
						} else if ((q.front()).first < d) {
							while (!q.empty()) {
								q.pop();
							}
							P p;
							p.first = d;
							p.second = j;
							q.push(p);
						} else if ((q.front()).first == d) {
							P p;
							p.first = d;
							p.second = j;
							q.push(p);
						}
					}
				}
				if (q.empty()) {
					for (int j = 0; j < n; j++) {
						if (isu[j] == '#') {
							isu[j] = 'D';
							break;
						}
					}
				} else {
					P p = q.front();
					q.pop();
					isu[p.second] = 'D';
				}
			}
		}
		for (int i = 0; i < n; i++) {
			cout << isu[i];
		}
		cout << endl;
	}
	return 0; 
}

C++: #include<bits/stdc++.h>

#define REP(i,s,n) for(int i=s;i<n;i++)
#define rep(i,n) REP(i,0,n)

using namespace std;

char tmp[200];

int main(){
  int n,m;
  while( cin >> n >> m, n|m ){
    rep(i,n) tmp[i] = '#';
    rep(_,m){
      char c;
      cin >> c;
      if( c == 'A' ){
        rep(i,n) if( tmp[i] == '#' ) {
          tmp[i] = c;
          break;
        }
      } else if( c == 'B' ) {
        bool ok = false;
        for(int i=n-1;i>=0;i--) if( tmp[i] == '#' ) {
          int res = 0;
          if( 0 <= i-1 ) res += (tmp[i-1]=='A');
          if( i+1 < n  ) res += (tmp[i+1]=='A');
          if( res ) continue;
          tmp[i] = c;
          ok = true;
          break;
        }
        if( !ok ) {
          rep(i,n) if( tmp[i] == '#' ) {
            tmp[i] = c;
            break;
          }
        }
      } else if( c == 'C' ) {
        bool failed = false;
        rep(i,n) if(tmp[i]!='#') {
          failed = true;
          break;
        }
        if( !failed ) {
          tmp[(n&1)?((n+1)/2):(n/2+1)] = c;
        } else {
          rep(i,n) if( tmp[i] != '#' ){
            if( i+1 < n && tmp[i+1] == '#' ){
              tmp[i+1] = c;
              break;
            }
            if( 0 <= i-1 && tmp[i-1] == '#' ){
              tmp[i-1] = c;
              break;
            }
          }
        }
      } else {
        bool flag = true;
        rep(i,n) if( tmp[i] != '#' ) {
          flag = false;
          break;
        }
        if( flag ){
          tmp[0] = c;
        } else {
          int pos = -1, maxi = -1;
          rep(i,n)if(tmp[i]=='#'){
            int cost = 0, x = i;          
            while( 0 <= x-1 ) {
              x--;
              if( tmp[x] != '#' ) break;
              cost++;
            }
            int cost2 = 0;
            x = i;
            while( x+1 < n ){
              x++;
              if( tmp[x] != '#' ) break;
              cost2++;
            }
            cost = min(cost,cost2);
            if( cost > maxi ){
              maxi = cost;
              pos = i;
            }
          }
          assert( pos != -1 );
          tmp[pos] = c;
        }
      }
    }
    rep(i,n) cout << tmp[i]; cout << endl;
  }
  return 0;
}