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<bits/stdc++.h>
using namespace std;

int main(){

  while(1){

    int n,m;

    cin >> n >> m;

    if(n==0 && m == 0) break;

    char ch[n];

    for(int i=0;i<n;i++){
      ch[i] = '#';
    }

    for(int k=0;k<m;k++){
      char c;

      cin >> c;

      if(c=='A'){
        for(int i=0;i<n;i++){
          if(ch[i]=='#'){
            ch[i] = 'A';
            break;
          }
        }
      }
      else if(c == 'B'){
        bool flag = true;
        for(int i=n-1;i!=0;i--){
          if(ch[i]=='#'){
            if(i == n-1 && ch[i-1]!='A'){
              ch[i] = 'B';
              flag = false;
              break;
            }
            else if(i == 0 && ch[i+1]!='A'){
              ch[i] = 'B';
              flag = false;
              break;
            }
            if((ch[i+1]!='A') && (ch[i-1]!='A')){
              ch[i] = 'B';
              flag = false;
              break;
            }
          }
        }
        if(flag){
          for(int i=0;i<n;i++){
            if(ch[i]=='#'){
              ch[i] = 'B';
              break;
            }
          }
        }
      }
      else if(c == 'C'){
        if(k==0){
          if(n%2==0){
            ch[n/2+1-1] = 'C';
          }
          else{
            ch[(n+1)/2 -1] = 'C';
          }
        }
        else{
          for(int i=0;i<n;i++){
            if(ch[i]!='#'){
              if(ch[i+1]=='#'){
                ch[i+1] = 'C';
                break;
              }
              else if(i!=0 && ch[i-1]=='#'){
                ch[i-1] = 'C';
                break;
              }
              else{
                for(int j=i;j<n;j++){
                  if(ch[j]=='#'){
                    ch[j] = 'C';
                    break;
                  }
                }
                break;
              }
            }
          }
        }
      }
      else{
        int dis[n];
        int mdis = 0,pos = 0;
        if(k==0){
          ch[0] = 'D';
        }
        else{
          for(int i=0;i<n;i++){
            if(ch[i]!='#'){
              dis[0] = i;
              break;
            }
          }
          for(int i=1;i<n;i++){
            if(ch[i] != '#'){
              dis[i] = 0;
              continue;
            }
            dis[i] = dis[i-1] + 1;
            for(int j=i+1;j<n;j++){
              if(ch[j] != '#'){
                dis[i] = min(dis[i],j-i);
              }
            }
          }
          for(int i=0;i<n;i++){
            if(dis[i] > mdis){
              mdis = dis[i];
              pos = i;
            }
          }
          ch[pos] = 'D';
        }
      }
    }

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

  }
}

C++: #include<bits/stdc++.h>
using namespace std;
typedef long long int lld;
#define INF 100000
int main(){
  int n,m;
  while(cin >> n >> m , n || m){
    string str = "";
    for(int i=0;i<n;i++) str += '#';
    for(int c=0;c<m;c++){
      char h; cin >> h;
      if(h == 'A'){
	for(int i=0;i<n;i++){
	  if(str[i] == '#'){
	    str[i] = 'A';
	    break;
	  }
	}
      } else if(h == 'B'){
	int first = -1;
	bool flg = true;
	for(int i=n-1;i>=0;i--){
	  if(first == -1 && str[i] == '#') first = i;
	  if(str[i] == '#' && str[i-1] != 'A' && str[i+1] != 'A'){
	    str[i] = 'B';
	    flg = false;
	    break;
	  }
	  if(flg){
	    for(int i=0;i<n;i++){
	      if(str[i] == '#'){
		str[i] = 'B';
		break;
	      }
	    }	
	  }
	}
      } else if(h == 'C'){
	if(c == 0) str[(n%2==1 ? (n+1)/2 : (n/2+1)) -1] = 'C';
	else {
	  for(int i=0;i<n;i++){
	    if(str[i] != '#'){
	      if(str[i+1] == '#') {
		str[i+1] = 'C';
		break;
	      }
	      if(str[i-1] == '#') {
		str[i-1] = 'C';
		break;
	      }
	    }
	  }
	}
      } else {
	int k = (c==0 ? 0 : 1);
	if(!k){
	  str[0] = 'D';
	} else {
	  int MAX=0,j,m,MAXi;
	  for(int i=0;i<n;i++){
	    if(str[i]=='#'){
	      for(j=i-1;j>=0;j--) if(str[j]!='#') break;
	      for(m=i+1;m<n;m++) if(str[m]!='#') break;
	      if(j==-1) j = INF;
	      if(m==n) m = INF;
	      if(j==INF){
		if(k-i > MAX) MAX = m-i,MAXi=i,MAXi=i;
	      } else if(m==INF){
		if(i-j > MAX) MAX = i-j,MAXi=i;
	      } else{
		if(min(m-i,i-j) > MAX) MAX=min(m-i,i-j),MAXi=i;
	      }
	    }
	  }
	  if(MAX == 0) {
	    for(int i=0;i<n;i++){
	      if(str[i] == '#'){
		str[i] = h;
		break;
	      }
	    }
	  }
	  else str[MAXi] = 'D';
	}
      }
    }
    cout << str << endl;
  }
}