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;

int main(){
  int n,m;
  while(cin >> n >> m , n){
    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] = h;
	    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(i == n-1 && str[i] == '#' && str[i-1] != 'A'){
	    str[i] = h;
	    flg = false;
	    break;
	  } else if(i == 0 && str[i] == '#' && str[i+1] != 'A'){
	    str[i] = h;
	    flg = false;
	    break;*/
	  if(str[i] == '#' && str[i-1] != 'A' && str[i+1] != 'A'){
	    str[i] = h;
	    flg = false;
	    break;
	  }
	  if(flg) str[first] = h;
	}
      } 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] = h;
		break;
	      }
	      if(str[i-1] == '#') {
		str[i-1] = h;
		break;
	      }
	    }
	  }
	}
      } else {
	int first = -1;
	int idx = -1;
	int maxx = 0;
	for(int i=0;i<n;i++){
	  if(first == -1 && str[i] == '#') first = i;
	  if(str[i] != '#'){
	    int now = i;
	    int cnt = 0;
	    while(i+1 < n && str[i+1] == '#'){
	      if(first == -1 && str[i+1] == '#') first = i+1;
	      cnt++;
	      i++;
	    }
	    if(i+1 == n) {
	      if(cnt > maxx){
		maxx = cnt;
		idx = now + cnt;
	      }
	    }
	    else if(cnt > 2){
	      cnt = (cnt-1)/2;
	      if(cnt > maxx){
		maxx = cnt;
		idx = now + cnt + 1;
	      }
	    }
	  }
	}
	if(idx == -1) str[first] = h;
	else str[idx] = h;
      }
    }
    cout << str << endl;
  }
}

C++: #include <iostream>
#include <string>

using namespace std;

int main(){
	while(true){
		int n, m;
		cin >> n >> m;
		if(n == 0 && m == 0){ break; }
		char chairs[101];
		for(int i = 0; i < n; i++){ chairs[i] = '#'; }
		chairs[n] = '\0';
		for(int i = 0; i < m; i++){
			string s;
			cin >> s;
			bool flag = false;
			if(s[0] == 'A'){
				for(int j = 0; j < n; j++){
					if(chairs[j] == '#'){ chairs[j] = 'A'; break; }
				}
			}else if(s[0] == 'B'){
				bool flag = false;
				for(int j = n - 1; j >= 0; j--){
					if(chairs[j] != '#'){ continue; }
					if((j < 1 || chairs[j - 1] != 'A') && chairs[j + 1] != 'A'){
						chairs[j] = 'B';
						flag = true;
						break;
					}
				}
				if(!flag){
					for(int j = 0; j < n; j++){
						if(chairs[j] == '#'){ chairs[j] = 'B'; break; }
					}
				}
			}else if(s[0] == 'C'){
				if(i == 0){
					chairs[n / 2] = 'C';
				}else{
					int j = 0;
					for(; j < n; j++){
						if(chairs[j] != '#'){ break; }
					}
					if(chairs[j + 1] == '#'){
						chairs[j + 1] = 'C';
					}else if(j > 0 && chairs[j - 1] == '#'){
						chairs[j - 1] = 'C';
					}else{
						for(; j < n; j++){
							if(chairs[j] == '#'){ break; }
						}
						chairs[j] = 'C';
					}
				}
			}else if(s[0] == 'D'){
				if(i == 0){
					chairs[0] = 'D';
				}else{
					int dist = 0, idx = 0;
					for(int j = 0; j < n; j++){
						if(chairs[j] != '#'){ continue; }
						int d = 100;
						for(int k = j - 1; k >= 0; k--){
							if(chairs[k] != '#'){ d = min(d, j - k); break; }
						}
						for(int k = j + 1; k < n; k++){
							if(chairs[k] != '#'){ d = min(d, k - j); break; }
						}
						if(d > dist){ idx = j; dist = d; }
					}
					chairs[idx] = 'D';
				}
			}
		}
		cout << chairs << endl;
	}
	return 0;
}

C++: #define _USE_MATH_DEFINES
#include<cstdio>
#include<iostream>
#include<vector>
#include<algorithm>
#include<queue>
#include<stack>
#include<cmath>
#include<climits>
using namespace std;

int N, M;
char C;
char chairs[102];

void PutA(){
	for(int i = 1; i <= N; i++){
		if(chairs[i] == '#'){
			chairs[i] = 'A';
			return;
		}
	}
}

void PutB(){
	for(int i = N; i >= 1; i--){
		if(chairs[i+1] != 'A' && chairs[i] == '#' && chairs[i-1] != 'A'){
			chairs[i] = 'B';
			return;
		}
	}
	for(int i = 1; i <= N; i++){
		if(chairs[i] == '#'){
			chairs[i] = 'B';
			return;
		}
	}
}

void PutC(){
	for(int i = 1; i <= N; i++){
		if(chairs[i] != '#'){
			if(chairs[i+1] == '#'){
				chairs[i+1] = 'C';
				return;
			}
			if(chairs[i-1] == '#'){
				chairs[i-1] = 'C';
				return;
			}
		}
	}
	if(N&1) chairs[ (N+1)/2 ] = 'C';
	chairs[N/2 + 1] = 'C';
}

void PutD(){
	int l[102], r[102];
	l[0] = INT_MAX/2; r[N+1] = INT_MAX/2;
	for(int i = 1; i <= N; i++) l[i] = (chairs[i] == '#') ? l[i-1] + 1 : 0;
	for(int i = N; i >= 1; i--) r[i] = (chairs[i] == '#') ? r[i+1] + 1 : 0;

	int ma_idx, ma_d = -1;
	for(int i = 1; i <= N; i++){
		int d = min(l[i], r[i]);
		if(d >= INT_MAX/2) d = INT_MAX;
		if(ma_d < d){
			ma_idx = i; ma_d = d;
		}
	}
	chairs[ma_idx] = 'D';
}

int main(){
	void (*put[4])() = {PutA, PutB, PutC, PutD};

	while(cin >> N >> M, N || M){
		fill_n(chairs, N+2, '#');
		chairs[0] = chairs[N+1] = '.';
		for(int i = 0; i < M; i++){
			cin >> C;
			put[C - 'A']();
			//for(int i = 1; i <= N; i++) printf("%c", chairs[i]); puts("");
		}
		for(int i = 1; i <= N; i++) printf("%c", chairs[i]); puts("");
	}
	return 0;
}