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
		}
		chairs := make([]string, n)
		for i := range chairs {
			chairs[i] = "#"
		}
		sit := func(who string) {
			if who == "A" {
				chairs[getStringIndex(chairs, "#")] = "A" // A国の人は左から見て行って空いている席を見つけ次第座る
				// log.Println(chairs)
			} else if who == "B" {
				f := true
				for i := n-1; i > -1; i-- {
					// B国の人はA国人の隣以外で、右端から空いている椅子に座る
					if chairs[i] == "#" && (i == 0 || chairs[i-1] != "A") && (i == n-1 || chairs[i+1] != "A") {
								chairs[i] = "B"
								f = false
								break
					}
				}
				// A国人の隣しか空いていない場合、我慢して左端から空いている椅子に座る
				if f {
					chairs[getStringIndex(chairs, "#")] = "B"
				}
				// log.Println(chairs)
			} else if who == "C" { // C国人は人の隣に座りたい
				for i := 0; i < n; i++ {
					if chairs[i] != "#" {
						if i + 1 <= n-1 && chairs[i+1] == "#" {
							chairs[i+1] = "C"
							break
						} else if i - 1 >= 0 && chairs[i-1] == "#" {
							chairs[i-1] = "C"
							break
						}
					}
				}
				// log.Println(chairs)
			} else { // who == "D"
				// D国の人は人から離れて座りたい
				// どうしても誰かの隣に座らなければならない場合、その中で一番左側にある椅子に座る
				scores := make([]int, n)
				for i := range scores {
					scores[i] = INF
				}
				for i, v := range chairs {
					if v == "#" {
						scores[i]++
					}
				}
				for i := n-1; i > -1; i-- {
					tmp := 0
					if chairs[i] == "#" {
						tmp++
					} else {
						tmp = 0
					}
					scores[i] = min(scores[i], tmp)
				}
				chairs[getIntIndex(scores, getMax(scores))] = "D"
				// log.Println(chairs)
			}
		}
		scanner.Scan()
		if first := scanner.Text(); first == "A" || first == "D" {
			chairs[0] = first
		} else if first == "B" {
			chairs[len(chairs)-1] = "B"
		} else {
			chairs[n/2] = "C"
		}
		// log.Println(chairs)
		for i := 0; i < m-1; i++ {
			scanner.Scan()
			sit(scanner.Text())
		}
		fmt.Println(strings.Join(chairs, ""))
	}
}

func getStringIndex(chairs []string, who string) int {
	for i, v := range chairs {
		if v == who {
			return i
		}
	}
	return -1
}

func getIntIndex(scores []int, target int) int {
	for i, v := range scores {
		if v == target {
			return i
		}
	}
	return -1
}

func min(a, b int) int {
	if a > b {
		return b
	}
	return a
}

func getMax(scores []int) int {
	tmp := -1
	for _, v := range scores {
		if tmp < v {
			tmp = v
		}
	}
	return tmp
}

C++: #include<iostream>
using namespace std;
#define REP(i,b,n) for(int i=b;i<n;i++)
#define rep(i,n)   REP(i,0,n)

int findlm(int n,char *in){
  rep(i,n)if(in[i] == 0)return i;
}

int findNotA(int n,char *in){
  for(int i=n-1;i>=0;i--){
    if (in[i] == 0 && (i==0||in[i-1]!= 'A') && (i == n-1 || in[i+1] != 'A'))return i;
  }
  return -1;
}

int nextP(int n,char *in){
  rep(i,n){
    if (in[i] != 0){
      if (i < n-1 && in[i+1] == 0)return i+1;
      if (i >0 && in[i-1] == 0)return i-1;
    }
  }
  return -1;
}

int notNextP(int n,char *in){
  int ret = -1,maxdist=0;
  rep(i,n){
    if( in[i] != 0)continue;
    int tmpdist = 0;
    for(int j=i-1;j>=0;j--){
      if (in[j] != 0){tmpdist=i-j;break;}
    }
    for(int j=i+1;j<n;j++){
      if (in[j] != 0){tmpdist=max(tmpdist,j-i);break;}
    }
    if (tmpdist > maxdist)ret=i,maxdist = tmpdist;
  }
  return ret;
}

void put(int n,char *in,char inp){
  int index = -1;
  if (inp == 'A'){
    index = findlm(n,in);
  }else if (inp == 'B'){
    index = findNotA(n,in);
    if (index == -1)index = findlm(n,in);
  }else if (inp == 'C'){
    index = nextP(n,in);
    if (index == -1)index = n%2 == 0?(n)/2:n/2;
  }else if (inp == 'D'){
    index = notNextP(n,in);
    if (index == -1)index = 0;
  }
  in[index] = inp;
}


main(){
  char op[255];
  op['A']='A';  op['B']='B';  op['C']='C';  op['D']='D';
  op[0]='#';

  int n,m;
  char in[100];
  while(cin>>n>>m && n){
    rep(i,n)in[i] = 0;
    rep(i,m){
      char inp;
      cin>>inp;
      put(n,in,inp);
    }
    rep(i,n)cout << op[in[i]] ;
    cout << endl;
  }


}

C++: #include <string>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
void solve1(string& s, char c) {
	s[s.find('#')] = c;
}
void solve2(string& s) {
	for (int i = s.size() - 1; i >= 0; i--) {
		if (s[i] != '#' && (i < s.size() - 1 && s[i + 1] != 'A') && (i > 0 && s[i - 1] != 'A')) {
			s[i] = 'B'; return;
		}
	}
	solve1(s, 'B');
}
vector<int> dist(string& s) {
	vector<int> ret(s.size(), 999999999);
	for (int i = 0; i < s.size(); i++) {
		if (s[i] != '#') {
			for (int j = 0; j < s.size(); j++) {
				ret[j] = min(ret[j], abs(i - j));
			}
		}
	}
	return ret;
}
void solve3(string &s) {
	for (int i = 0; i < s.size(); i++) {
		if (s[i] != '#') {
			if (i < s.size() - 1 && s[i + 1] == '#') {
				s[i + 1] = 'C'; return;
			}
			if (i > 0 && s[i - 1] == '#') {
				s[i - 1] = 'C'; return;
			}
		}
	}
	if (s.size() % 2 == 1) s[(s.size() - 1) / 2] = 'C';
	else s[s.size() / 2] = 'C';
}
void solve4(string &s) {
	vector<int> ret = dist(s);
	s[max_element(ret.begin(), ret.end()) - ret.begin()] = 'D';
}
int N, Q;
int main() {
	while (cin >> N >> Q, N) {
		string s(N, '#'), t;
		while (Q--) {
			cin >> t;
			if (t == "A") solve1(s, 'A');
			if (t == "B") solve2(s);
			if (t == "C") solve3(s);
			if (t == "D") solve4(s);
		}
		cout << s << endl;
	}
	return 0;
}

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

#define MAX 100

int n,m;
char chair[MAX+2];
  
void A()
{
    for (int i = 1; i <= n; i++) {
	if (chair[i] == '#') {
	    chair[i] = 'A';
	    return;
	}
    }
}
 
void B()
{
    int pos = -1;
    for (int i = 1; i <= n; i++) {
	if (chair[i] == 'A') {
	    pos = i;
	    break;
	}
    }
   
    if (pos == -1) {
	for (int i = n; i > 0; i--) {
	    if (chair[i] == '#') {
		chair[i] = 'B';
		return;
	    } 
	}
    } else {
	for (int i = n; i > 0; i--) {
	    if (chair[i] == '#' && chair[i-1] != 'A' && chair[i+1] != 'A') {
		chair[i] = 'B';
		return;
	    }
	}
  
	for (int i = 1; i <= n; i++) {
	    if (chair[i] == '#') {
		chair[i] = 'B';
		return;
	    }
	}
    }
}
  
void C()
{
    int cnt = count(chair, chair+n, '#');
  
    if (cnt == n) {
	if (n % 2) {
	    chair[(n+1)/2] = 'C';
	} else {
	    chair[n/2+1] = 'C';
	}
    } else {
	for (int i = 1; i <= n; i++) {
	    if (chair[i] != '#' && chair[i+1] == '#' && i != n) {
		chair[i+1] = 'C';
		return;
	    }
	    if (chair[i] != '#' && chair[i-1] == '#' && i != 1) {
		chair[i-1] = 'C';
		return;
	    }
	}
    }
}
  
void D()
{
    int Breadth = 0, pos;
    int cnt = count(chair, chair+n, '#');
 
    if (cnt == n) {
	chair[1] = 'D';
	return;
    } 
 
    for (int i = 1; i <= n; i++) {
	int dist = (1<<29);
	if (chair[i] != '#') continue;
	for (int j = 1; j <= n; j++) {
	    if (chair[j] != '#') {
		dist = min(dist, abs(j-i)+1);
	    }
	}
	if (Breadth < dist) {
	    Breadth = dist;
	    pos = i;
	} 
    }
  
    chair[pos] = 'D';
}
  
int main()
{
    while (cin >> n >> m, n | m) {
	char person;
  
	for (int i = 0; i <= n+1; i++) {
	    chair[i] = '#';
	}
  
	for (int i = 0; i < m; i++) {
	    cin >> person;
       
	    switch (person) {
                case 'A': 
                    A(); 
                    break;
                case 'B': 
                    B(); 
                    break;
                case 'C': 
                    C(); 
                    break;
                case 'D': 
                    D(); 
                    break;
	    }
	}
	for (int i = 1; i <= n; i++) {
	    cout << chair[i];
	}
	cout << endl;
    }
    return 0;
}