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 <stdio.h>
#include <cmath>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>

using namespace std;

struct Info{
	Info(){
		country = '#';
	}
	char country;
};

void func(int n,int m){
	Info info[n];
	char tmp[2];
	int last_space,maximum,most_left_index,pre_index,div;
	bool FLG;

	for(int loop = 0; loop < m; loop++){
		scanf("%s",tmp);

		switch(tmp[0]){
		case 'A':
			for(int i = 0; i < n; i++){
				if(info[i].country == '#'){
					info[i].country = 'A';
					break;
				}
			}
			break;
		case 'B':
			last_space = -1;
			FLG = false;

			if(info[n-1].country == '#' && info[n-1].country != 'A'){
				info[n-1].country = 'B';
				break;
			}

			for(int i = n-2; i >= 1; i--){
				if(info[i].country == '#'){
					last_space = i;
					if(info[i-1].country != 'A' && info[i+1].country != 'A'){
						info[i].country = 'B';
						FLG = true;
						break;
					}
				}
			}

			if(FLG)break;

			if(info[0].country == '#'){
				last_space = 0;
				if(info[1].country != 'A'){
					info[0].country = 'B';
					break;
				}
			}

			info[last_space].country = 'B';
			break;
		case 'C':
			if(loop == 0){
				if(n%2 == 1){
					info[(n+1)/2].country = 'C';
				}else{
					info[(n/2)+1].country = 'C';
				}
			}else{
				FLG = false;
				if(info[0].country != '#' && info[1].country == '#'){
					info[1].country = 'C';
					break;
				}
				for(int i = 1; i <= n-2; i++){
					if(info[i].country != '#'){
						if(info[i+1].country == '#'){
							info[i+1].country = 'C';
							FLG = true;
							break;
						}else if(info[i-1].country == '#'){
							info[i-1].country = 'C';
							FLG = true;
							break;
						}
					}
				}
				if(FLG)break;

				if(info[n-1].country != '#' && info[n-2].country == '#'){
					info[n-2].country = 'C';
				}
			}
			break;
		case 'D':

			if(loop == 0){
				info[0].country = 'D';
			}else{
				maximum = 0;
				most_left_index = -1;
				pre_index = -1;

				if(info[0].country == '#'){
					most_left_index = 0;
					for(div = 1; div <= n-1; div++){
						if(info[div].country != '#'){
							break;
						}
					}
					maximum = div;
				}else{
					pre_index = 0;
				}

				for(int i = 1; i <= n-1; i++){
					if(info[i].country == '#'){

						if(pre_index == -1){
							//Do nothing
						}else{
							if(i - pre_index > maximum){
								maximum = i - pre_index;
								most_left_index = i;
							}
						}

						for(div = 1; i+div <= n-1; div++){
							if(info[i+div].country != '#'){
								break;
							}
						}
						if(div > maximum){
							maximum = div;
							most_left_index = i;
						}
						pre_index = i;
					}
				}

				info[most_left_index].country = 'D';
			}

			break;
		}
	}

	for(int i = 0; i < n; i++)printf("%c",info[i].country);
	printf("\n");
}

int main(){

	int n,m;
	while(true){
		scanf("%d %d",&n,&m);
		if(n == 0 && m == 0)break;

		func(n,m);

	}

    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int n,m;
char chair[200]={};

void A(char ch){
  int i=1;
  while(chair[i]!='#') i++;
  chair[i]=ch;
}

void B(){
  int i=n;
  while(chair[i]!='#')i--;
  chair[i]='B';
}

void C(int kai){
  if(kai==0){
    if(n%2)chair[(n+1)/2]='C';
    else chair[n/2+1]='C';
    return;
  }
  for(int i=1;i<=n;i++){
    if(chair[i]=='#')continue;
    if(chair[i+1]=='#') chair[i+1]='C',i=n;
    if(chair[i-1]=='#') chair[i-1]='C',i=n;

  }
}

void D(int kai){
  int R=0,L=0;
  while(chair[n-R]=='#')R++;
  while(chair[L+1]=='#') L++;
  
  int M=0,idx=0,cnt=0;
  for(int i=1;i<=n;i++){
    if(chair[i]=='#')cnt++;
    else cnt=0;
    if(M+M%2<cnt) M=cnt,idx=i;
  }
  if(M<=2&&L<=1&&R<=1) {
    A('D');
    return;
  }
   if(L-1>=(M-1)/2&&L-1>=R-1)chair[1]='D';
  else if(R-1>=(M-1)/2)chair[n]='D';
  else chair[idx-M+1+(M-1)/2]='D';

}

int main(){

  while(1){
    cin>>n>>m;
    if(!n&&!m)break;
    for(int i=0;i<=130;i++) chair[i]='#';
    chair[0]=chair[n+1]=0;
    for(int i=0;i<m;i++){
      char a;
      cin>>a;
      if(a=='A') A(a);
      if(a=='B') B();
      if(a=='C') C(i);
      if(a=='D') D(i);
    }
    printf("%s\n",chair+1);
  }
  return 0;
}

C++: #include<stdio.h>
#include<algorithm>
using namespace std;
char str[101];
char c[2];
int p[100];
int ABS(int a){return a<0?-a:a;}
int main(){
	int a,b;
	while(scanf("%d%d",&a,&b),a+b){
		for(int i=0;i<a;i++)str[i]='#';
		str[a]=0;
		bool ok;
		for(int i=0;i<b;i++){
			scanf("%s",c);
			switch(c[0]){
				case 'A':
					for(int j=0;j<a;j++)
						if(str[j]=='#'){
							str[j]='A';
							break;
						}
					break;
				case 'B':
					ok=false;
					for(int j=a-1;j>=0;j--){
						if(str[j]=='#'&&(j==0||str[j-1]!='A')&&str[j+1]!='A'){
							ok=true;
							str[j]='B';
							break;
						}
					}
					if(!ok){
						for(int j=0;j<a;j++)
							if(str[j]=='#'){
								str[j]='B';
								break;
							}
					}
					break;
				case 'C':
					if(i==0){
						str[a/2]='C';
					}else{
						ok=false;
						for(int j=0;j<a;j++){
							if(str[j]!='#')ok=true;
							if(ok&&str[j]=='#'){
								str[j]='C';
								break;
							}
						}
					}
					break;
				case 'D':
					for(int j=0;j<a;j++)p[j]=9999;
					for(int j=0;j<a;j++){
						if(str[j]!='#'){
							for(int k=0;k<a;k++)p[j]=min(p[j],ABS(j-k));
						}
					}
					int at=0;
					int cost=0;
					for(int j=0;j<a;j++){
						if(cost<p[j]){at=j;cost=p[j];}
					}
					str[at]='D';
					break;
			}
		}
		printf("%s\n",str);
	}
}