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, ""))
	}
}

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>
using namespace std;
int main(){
  while(true){
    int n,m,j;
    cin>>n>>m;
    if(!n&&!m)break;
    char seat[n+2];
    int len[n];
    bool sit = false;
    for(int i=0;i<n+2;i++){
      seat[i] = '#';
    }
    for(int i=0;i<m;i++){
      char p;
      cin>>p;
      if(p=='A'){
	for(j=1;j<n+1;j++){
	  if(seat[j]=='#'){
	    seat[j] = 'A';
	    break;
	  }
	}
	sit = true;
      }
      if(p=='B'){
	for(j=n;j>0;j--){
	  if(seat[j]=='#'&&seat[j-1]!='A'&&seat[j+1]!='A'){
	    seat[j] = 'B';
	    break;
	  }
	}
	if(j<0){
	  for(j=1;j<n+1;j++){
	    if(seat[j]=='#'){
	      seat[j] = 'B';
	      break;
	    }
	  }
	}
	sit = true;
      }
      if(p=='C'){
	if(!sit){
	  seat[n/2+1] = 'C';
	}
	else{
	  for(j=1;j<n;j++){
	    if(seat[j]!='#'){
	      if(seat[j+1]=='#'){
		seat[j+1] = 'C';
		break;
	      }
	      else if(seat[j-1]=='#'){
		seat[j-1] = 'C';
		break;
	      }
	    }
	  }
	}
	sit = true;
      }
      if(p=='D'){
	if(!sit){
	  seat[1] = 'D';
	}
	else{
	  int l=1000;
	  for(int j=1;j<n+1;j++){
	    if(seat[j]!='#')l = 0;
	    else l++;
	    len[j] = l;
	  }
	  l=1000;
	  int lenmax = 0,maxseat;
	  for(int j=n;j>0;j--){
	    if(seat[j]!='#')l = 0;
	    else l++;
	    len[j] = min(len[j],l);
	    if(lenmax<=len[j]){
	      lenmax=len[j];
	      maxseat=j;
	    }
	  }
	  seat[maxseat] = 'D';
	}
	sit = true;
      }
    }
    for(int i=1;i<n+1;i++)
      cout<<seat[i];
    cout<<endl;
  }
  return 0;
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<cstring>
#include<algorithm>
#include<map>
#include<set>
#include<cmath>
#include<cassert>
#include<queue>

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;

int ans[111];

int calc(int x, int n) {
	int l = 0;
	for (; l <= x; l++) {
		if (ans[x-l] != -1) break;
	}
	if (l == x+1) l = n;
	int r = 0;
	for (; r+x < n; r++) {
		if (ans[r+x] != -1) break;
	}
	if (r+x == n) r = n;
	return min(l, r);
}

int main() {
	int N, M;
	while (cin >> N >> M) {
		if (N == 0 && M == 0) break;
		memset(ans, -1, sizeof(ans));
		for (int t = 0; t < M; t++) {
			char c;
			cin >> c;
			if (c == 'A') {
				for (int i = 0; i < N; i++) {
					if (ans[i] == -1) {
						ans[i] = 0;
						break;
					}
				}
			}
			if (c == 'B') {
				bool done = false;
				for (int i = N-1; i >= 0; i--) if (ans[i] == -1) {
					if (ans[i+1] != 0) {
						if (i == 0 || ans[i-1] != 0) {
							done = true;
							ans[i] = 1;
							break;
						}
					}
				}
				if (!done) {
					for (int i = 0; i < N; i++) {
						if (ans[i] == -1) {
							ans[i] = 1;
							break;
						}
					}
				}
			}
			if (c == 'C') {
				if (t == 0) {
					ans[(N+1)/2] = 2;
				} else {
					for (int i = 0; i < N; i++) {
						if (ans[i] != -1) {
							if (ans[i+1] == -1) {
								ans[i+1] = 2;
								break;
							} else if (i > 0 && ans[i-1] == -1) {
								ans[i-1] = 2;
								break;
							}
						}
					}
				}
			}
			if (c == 'D') {
				if (t == 0) {
					ans[0] = 3;
				} else {
					int index = 0, maxi = 0;
					for (int i = 0; i < N; i++) if (ans[i] == -1) {
						int tmp = calc(i, N);
						if (tmp > maxi) {
							maxi = tmp;
							index = i;
						}
					}
					ans[index] = 3;
				}
			}
		}
		for (int i = 0; i < N; i++) {
			if (ans[i] == 0) cout << 'A';
			else if (ans[i] == 1) cout << 'B';
			else if (ans[i] == 2) cout << 'C';
			else if (ans[i] == 3) cout << 'D';
			else cout << '#';
		}
		cout << endl;
	}
	return 0;
}

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

int main(){

  int n,m;
  int j;
  char chair[100];
  char p;
  int left,max,m_left;

  while(1){

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

    for(int i=0; i<n; i++)
      chair[i]='#';


    for(int i=0; i<m; i++){

    cin >> p;

    if(p=='A'){
      j=0;
      while(chair[j]!='#')
	j++;

      chair[j]='A';
    }///////////////////////A

    else if(p=='B'){
      j=n-1;
      while(1){
	if(chair[j]=='#'){
	  if(j==n-1 && chair[j-1]!='A')
	    break;

	  else if(j>0 && chair[j+1]!='A' && chair[j-1]!='A')
	    break;

	  else if(chair[j+1]!='A')
	    break;
	  
	  else{
	    j=-1;
	    break;
	  }
	}
	j--;
      }

      if(j!=-1)
	chair[j]='B';

      else{
	j=0;
	while(chair[j]!='#')
	  j++;

	chair[j]='B';
      }

    }////////////////////////B


    else if(p=='C'){
      j=0;
      while(1){
	if(chair[j]!='#'){
	  if(chair[j+1]=='#'&&j!=n-1){
	    j++;
	    break;
	  }
	  else if(j!=0 && chair[j-1]=='#'){
	    j--;
	    break;
	  }
	}

	if(j==n-1){
	  j=-1;
	  break;
	}

	j++;
      }

      if(j==-1){
	if(n%2==1)
	  j=(n+1)/2;
	else
	  j=n/2+1;
      }

      chair[j] = 'C';
    }///////////////////////C


    else{
      left=0; max=1; m_left=0;
      j=1;
      while(j<=n-1){
	if(chair[j]!='#'){
	  left=j;

	  j++;
	}

	if(max < j-left){
	  max = j-left;
	  m_left = left;
	}

	j++;
      }

      if(max == 1)
	j = m_left+1;

      else if(max == n-1)
	j=0;

      else{
	if(max%2==1)
	  j=m_left+max/2 +1;
	else
	  j=m_left+max/2;
      }


      chair[j]='D';

    }///////////////////////D

    }


    for(int i=0; i<n; i++)
      cout << chair[i];


    cout << endl;

  }


  return 0;

}