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>
#include <string>

#define int long long

using namespace std;

class Solver {
};

signed main() {
	while (true) {
		int n, m;
		cin >> n >> m;
		if (n == 0)break;

		vector<char> status(n, '#');
		for (int i = 0; i < m; i++) {
			char who;
			cin >> who;
			if (who == 'A') {
				for (int i = 0; i < n; i++) {
					if (status[i] == '#') {
						status[i] = 'A';
						break;
					}
				}
			}
			if (who == 'B') {
				bool valid = false;
				for (int i = n - 1; i >= 0; i--) {
					if (status[i] != '#')continue;

					if ((i + 1 < n && status[i + 1] == 'A')
						|| (i - 1 >= 0 && status[i - 1] == 'A'))continue;

					status[i] = 'B';
					valid = true;
					break;
				}

				if (!valid) {
					for (int i = 0; i < n; i++) {
						if (status[i] == '#') {
							status[i] = 'B';
							break;
						}
					}
				}
			}
			if (who == 'C') {
				bool valid = false;
				for (int i = 0; i <n; i++) {
					if (status[i] == '#')continue;

					if (i + 1 < n && status[i + 1] == '#') {
						status[i + 1] = 'C';
						valid = true;
						break;
					}
					if (i - 1 >= 0 && status[i - 1] == '#') {
						status[i - 1] = 'C';
						valid = true;
						break;
					}
				}

				if (!valid) {
					status[n / 2] = 'C';
				}
			}
			if (who == 'D') {
				vector<int> left(n, 0);
				int last = -1;
				for (int i = 0; i < n; i++) {
					if (status[i] != '#') last = i;
					else {
						if (last >= 0) left[i] = i - last;
						else left[i] = 1e9;
					}
				}
				last = -1;
				vector<int> right(n, 0);
				for (int i = n - 1; i >= 0; i--) {
					if (status[i] != '#')last = i;
					else {
						if (last >= 0) right[i] = last - i;
						else right[i] = 1e9;
					}
				}
				int now = 0;
				int dist = -1;
				for (int i = 0; i < n; i++) {
					if (status[i] != '#')continue;

					if (min(left[i], right[i]) > dist) {
						now = i;
						dist = min(left[i], right[i]);
					}
				}
				status[now] = 'D';
			}
		}

		for (int i = 0; i < n; i++) {
			cout << status[i];
		}
		cout << endl;
	}
}

C++: #include <iostream>
#include <string>
#include <vector>
using namespace std;

int n, m;
string seat;

void sit(char a) {
    switch (a) {
        case 'A': {
            for (int i=1; i<=n; ++i) {
                if (seat[i] == '#') {
                    seat[i] = 'A'; return ;
                }
            }
            break;
        }
        case 'B': {
            for (int i=n; i>=1; --i) {
                if (seat[i] == '#' && seat[i-1] != 'A' && seat[i+1] != 'A') {
                    seat[i] = 'B'; return ;
                }
            }
            for (int i=1; i<=n; ++i) {
                if (seat[i] == '#') {
                    seat[i] = 'B'; return ;
                }
            }
            break;
        }
        case 'C': {
            bool empty = true;
            for (int i=1; i<=n; ++i) {
                if (seat[i] != '#') {
                    empty = false; break;
                }
            }
            if (empty) {
                seat[(n+2)/2] = 'C'; return ;
            }
            for (int i=1; i<=n; ++i) {
                if (seat[i] != '#') {
                    if (seat[i+1] == '#') {
                        seat[i+1] = 'C'; return ;
                    } else if (seat[i-1] == '#') {
                        seat[i-1] = 'C'; return ;
                    }
                }
            }
            break;
        }
        case 'D': {
            int k, mx = -1;
            for (int i=1; i<=n; ++i) {
                int l = 0;
                while (1 < i-l && seat[i-l] == '#') ++l;
                int r = 0;
                while (i+r < n && seat[i+r] == '#') ++r;
                if (mx < max(l,r)) {
                    k = i;
                    mx = max(l,r);
                }
            }
            seat[k] = 'D';
            break;
        }
    }
}

int main() {
    while (cin >> n >> m, (n || m)) {
        seat = "_";
        for (int i=0; i<n; ++i) {
            seat += "#";
        }
        seat += "_";
        char a;
        for (int i=0; i<m; ++i) {
            cin >> a;
            sit(a);
        }
        cout << seat.substr(1,n) << endl;
    }
    return 0;
}

C++: //08
#include<iostream>
#include<string>
#include<algorithm>

using namespace std;

int main(){
  for(int n,m;cin>>n>>m,n|m;){
    string c(n,'#');
    for(int i=0;i<m;i++){
      char p;
      cin>>p;
      switch(p){
      case 'B':
	int j;
	for(j=c.size()-1;j>=0;j--){
	  if(c[j]=='#'&&(j==0||c[j-1]!='A')&&(j==c.size()-1||c[j+1]!='A')){
	    c[j]=p;
	    break;
	  }
	}
	if(j>=0)break;
      case 'A':
	for(int j=0;j<c.size();j++){
	  if(c[j]=='#'){
	    c[j]=p;
	    break;
	  }
	}
	break;
      case 'C':
	if(i==0){
	  c[n/2]=p;
	}else{
	  for(int j=0;j<c.size();j++){
	    if(c[j]!='#'){
	      if(j!=c.size()-1&&c[j+1]=='#'){
		c[j+1]=p;
		break;
	      }else if(j!=0&&c[j-1]=='#'){
		c[j-1]=p;
		break;
	      }
	    }
	  }
	}
	break;
      case 'D':
	if(i==0){
	  c[0]=p;
	}else{
	  int b;
	  int d=-1;
	  for(int j=0;j<c.size();j++){
	    if(c[j]=='#'){
	      int cd;
	      int l;
	      for(l=j;l>=0&&c[l]!='#';l--);
	      cd=(l<0)?1000:j-l;
	      int r;
	      for(r=j;r<n&&c[r]!='#';r++);
	      cd=min(cd,(r==n)?1000:r-j);
	      if(d<cd){
		d=cd;
		b=j;
	      }
	    }
	  }
	  c[b]=p;
	}
	break;
      }
    }
    cout<<c<<endl;
  }
  return 0;
}