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 <iostream>
#include <string>
#include <cmath>
#include <algorithm>
//#include <set>

int n, m; //??????, ???

int main() {
    while(std::cin >> n >> m, n + m) {
        std::string str(n, '#');

        char q;
        for(int a = 0; a < m; ++a) {
            std::cin >> q;

            bool ok;
            switch(q) {
            case 'A':
                str[str.find("#")] = 'A';
                break;
            case 'B':
                ok = false;
                for(int i = n - 1; i >= 0; --i) {
                    if((i == 0 || str[i - 1] != 'A') && (i == n - 1 || str[i + 1] != 'A') && str[i] == '#') {
                        ok = true;
                        str[i] = 'B';
                        break;
                    }
                }
                if(!ok) {
                    str[str.find("#")] = 'B';
                }
                break;
            case 'C':
                ok = false;
                for(int i = 0; i < n; ++i) {
                    if(str[i] != '#') {
                        if(i != n - 1 && str[i + 1] == '#') {
                            str[i + 1] = 'C';
                            ok = true; break;
                        }
                        if(i != 0 && str[i - 1] == '#') {
                            str[i - 1] = 'C';
                            ok = true; break;
                        }
                    }
                }

                if(!ok) {
                    str[n / 2] = 'C';
                }
                break;
            case 'D':
                int dist[101];
                for(int i = 0; i < n; ++i) {
                    int d = 1e+9;
                    if(str[i] == '#') {
                        for(int j = 0; j < n; ++j) {
                            if(str[j] != '#') {
                                d = std::min(d, std::abs(i - j));
                            }
                        }
                    }else {
                        d = 0;
                    }
                    
                    dist[i] = d;
                }

                int maxIdx = 0;
                for(int i = 1; i < n; ++i) {
                    if(dist[i] > dist[maxIdx]) {
                        maxIdx = i;
                    }
                }

                str[maxIdx] = 'D';

                break;
            }
//            std::cout << str << std::endl;
        }

        std::cout << str << std::endl;
    }
}

C++: #include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <cmath>
#include <stack>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <list>
#include <iostream>
#include <sstream>
#include <climits>
#include <cfloat>
#include <complex>

using namespace std;

int main()
{
	int n, m;
	while (scanf("%d %d", &n, &m), n | m)
	{
		char t[102];	// 1~n
		memset(t, '#', sizeof(t));
		for (int w = 0; w < m; ++w)
		{
			char c;
			scanf(" %c", &c);
			if (c == 'A')
			{
				int i;
				for (i = 1; t[i] != '#'; ++i)
					;
				t[i] = 'A';
			}
			else if (c == 'B')
			{
				int i;
				for (i = n; i >= 1; --i)
					if (t[i-1] != 'A' && t[i] == '#' && t[i+1] != 'A')
						break;
				if (i == 0)
					for (i = 1; t[i] != '#'; ++i)
						;
				t[i] = 'B';
			}
			else if (c == 'C')
			{
				int i;
				if (w == 0)
					i = n / 2 + 1;
				else
				{
					for (i = 2; t[i] != '#'; ++i)
						;
					for (++i; t[i] != '#'; --i)
						;
				}
				t[i] = 'C';
			}
			else
			{
				int i;
				if (w == 0)
					i = 1;
				else
				{
					int dis[102];
					for (int j = 0; j < 102; ++j)
						dis[j] = 100000000;
					for (int j = 1; j <= n; ++j)
					{
						if (t[j] != '#')
							for (int k = 1; k <= n; ++k)
								dis[k] = min(dis[k], abs(j-k));
					}
					int d = 0;
					for (int j = 1; j <= n; ++j)
					{
						if (dis[j] > d)
						{
							i = j;
							d = dis[j];
						}
					}
				}
				t[i] = 'D';
			}
		}
		for (int i = 1; i <= n; ++i)
			putchar(t[i]);
		printf("\n");
	}

	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)
int main(){
	int n,m;
	while(cin>>n>>m,n){
		char input,chair[100];
		rep(i,n)chair[i]='#';
		rep(i,m){
			bool stop=false;
			cin>>input;
			if(input=='A'){
				rep(j,n)if(chair[j]=='#'){
					chair[j]='A';
					break;
				}
			}else if(input=='B'){
				rep(j,n)if(chair[j]=='#'&&(j==0||chair[j-1]!='A')&&(j==n-1||chair[j+1]=='A')){
					chair[j]='B';
					stop=true;
					break;
				}
				if(!stop)rep(j,n)if(chair[j]=='#'){
					chair[j]='B';
					break;
				}
			}else if(input=='C'){
				if(i==0)chair[n/2+1]='C';
				else rep(j,n)if(chair[j]=='#'&&(j==0||chair[j-1]!='#')&&(j==n-1||chair[j+1]!='#')){
					chair[j]='C';
					stop=true;
					break;
				}
				if(!stop)rep(j,n)if(chair[j]=='#'){
					chair[j]='C';
					break;
				}
			}else{
				rep(j,n)if(chair[j]=='#'&&(j==0||chair[j-1]=='#')&&(j==n-1||chair[j+1]=='#')){
					chair[j]='D';
					stop=true;
					break;
				}
				if(!stop)rep(j,n)if(chair[j]=='#'){
					chair[j]='D';
					break;
				}
			}
		}
		rep(i,n)cout<<chair[i];
		cout<<endl;
	}
}