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;
typedef long long int lld;
#define INF 100000

int n,m;
string str;

void SetA(int num){
  for(int i=0;i<n;i++){
    if(str[i] == '#'){
      if(num == 1) str[i] = 'A';
      else if(num == 2) str[i] = 'B';
      else str[i] = 'D';
      return;
    }
  }
}

void SetB(){
  for(int i=n-1;i>=0;i--){
    if(str[i] == '#' && str[i-1] != 'A' && str[i+1] != 'A'){
      str[i] = 'B';
      return;
    }
  }
  SetA(2);
}

void SetC(int c){
  if(c == 0) {
    str[(n%2==1 ? (n+1)/2 : (n/2+1)) -1] = 'C';
    return;
  }
  for(int i=0;i<n;i++){
    if(str[i] != '#'){
      if(str[i+1] == '#') {
	str[i+1] = 'C';
	return;
      }
      if(str[i-1] == '#') {
	str[i-1] = 'C';
	return;
      }
    }
  } 
}

void SetD(int c){
  if(c == 0){
    str[0] = 'D';
    return;
  }
  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(c-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) SetA(3);
  else str[MAXi] = 'D';
}
  
int main(){
  while(cin >> n >> m , n || m){
    str = "";
    for(int i=0;i<n;i++) str += '#';
    for(int c=0;c<m;c++){
      char h; cin >> h;
      if(h == 'A') SetA(1);
      else if(h == 'B') SetB();
      else if(h == 'C') SetC((c==0 ? 0 : 1));
      else SetD((c==0 ? 0 : 1));
    }
    cout << str << endl;
  }
}

C++: #include <bits/stdc++.h>
using namespace std;

const int INF = (1 << 30) - 1;

int N, M;
char seat[105];

int left_insert()
{
    int k = 1;
    while ( k <= N && seat[k] != '#') {
        k++;
    }
    return ( k );
}

int main()
{

    while ( cin >> N >> M, N || M ) {

        fill_n(seat, 105, '#');
        for ( int i = 0; i < M; i++ ) {
            char c;
            bool flag = false;
            int d[105];
            int k;

            cin >> c;
            fill_n(d, 105, INF);
            switch ( c ) {
                case 'A' :
                    seat[ left_insert() ] = 'A';
                    break;
                case 'B' :
                    k = N;
                    while ( k > 0 && (seat[k] != '#' || seat[k - 1] == 'A' || seat[k + 1] == 'A') ) {
                        k--;
                    }
                    if ( k > 0 ) {
                        seat[k] = 'B';
                    } else {
                        seat[ left_insert() ] = 'B';
                    }
                    break;
                case 'C' :
                    for ( int i = 1; i <= N; i++ ) {
                        if ( seat[i] != '#' ) {
                            if ( i < N && seat[i + 1] == '#' ) {
                                seat[i + 1] = 'C';
                                flag = true;
                                break;
                            }
                            if ( i > 1 && seat[i - 1] == '#' ) {
                                seat[i - 1] = 'C';
                                flag = true;
                                break;
                            }
                        }
                    }
                    if ( !flag ) {
                        seat[N / 2 + 1] = 'C';
                    }
                    break;
                case 'D' :
                    for ( int i = 1; i <= N; i++ ) {
                        if ( seat[i] != '#' ) {
                            d[i] = 0;
                        }
                        d[i] = min(d[i - 1] + 1, d[i]);
                    }
                    for ( int i = N; i > 0; i-- ) {
                        d[i] = min(d[i], d[i + 1] + 1);
                    }
                    int maxp = 1;
                    for ( int i = 2; i <= N; i++ ) {
                        if ( d[i] > d[maxp] ) {
                            maxp = i;
                        }
                    }
                    seat[maxp] = 'D';
                    break;
            }
        }
        for ( int i = 1; i <= N; i++ ) {
            cout << seat[i];
        }
        cout << endl;
    }
}