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<stdio.h>
int main()
{
	int n,m,i,j,k;
	char c;
	while(scanf("%d%d",&n,&m),n)
	{
		int a[100]={0};	//0:emptyツ、{1,2,3,4}={A,B,C,D}
		for(k=0;k<m;++k)
		{
			scanf(" %c",&c);
			if(c=='A')
			{
				for(i=0;i<n;++i)if(!a[i]){a[i]=1;break;}
			}
			if(c=='B')
			{
				for(i=n-1;i>=0;--i)if(!a[i]){a[i]=2;break;}
			}
			if(c=='C')
			{
				if(!k)a[n/2]=3;
				else
				{
					for(i=0;i<n;++i)
					{
						if(i!=n-1&&a[i]&&!a[i+1]){a[i+1]=3;break;}
						if(i&&a[i]&&!a[i-1]){a[i-1]=3;break;}
					}
				}
			}
			if(c=='D')
			{
				if(!k)a[0]=4;
				else
				{
					int mc=-1,mi=-1;
					int cc=999;
					for(i=0;i<n;++i)
					{
						for(j=i;j>=0;--j)if(a[j]){if(cc>i-j)cc=i-j;break;}
						for(j=i;j<n;++j)if(a[j]){if(cc<j-i)cc=j-i;break;}
						if(mc<cc)mc=cc,mi=i;
					}
					a[mi]=4;
				}
			}
		}
		for(i=0;i<n;++i)putchar("#ABCD"[a[i]]);
		puts("");
	}
	return 0;
}

C++: #include <string>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
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;
		}
	}
	s[s.find('#')] = 'B';
}
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;
			}
		}
	}
	s[s.size() / 2] = 'C';
}
void solve4(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));
			}
		}
	}
	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") s[s.find('#')] = 'A';
			if (t == "B") solve2(s);
			if (t == "C") solve3(s);
			if (t == "D") solve4(s);
		}
		cout << s << endl;
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
long n,m,i,j,d[101],e[101],k,dd;
char c,a[101];
while(cin>>n){
cin>>m;
if(n==0)break;
for(i=0;i<n;i++)a[i]='#';
for(j=0;j<m;j++){
cin>>c;
if(c=='A'){
for(i=0;i<n;i++)if(a[i]=='#'){a[i]='A';break;}
}else if(c=='B'){
for(i=n-1;i>=0;i--){
if(a[i]!='#'||0<i&&a[i-1]=='A'||i<n-1&&a[i+1]=='A')continue;
a[i]='B';break;
}
if(i<0){for(i=0;i<n;i++)if(a[i]=='#'){a[i]='B';break;}}
}else if(c=='C'){
for(i=0;i<n;i++){
if(a[i]!='#'){
if(i<n-1&&a[i+1]=='#'){a[i+1]='C';break;}
else if(i>0&&a[i-1]=='#'){a[i-1]='C';break;}
}
}
if(i==n)a[n/2]='C';
}else if(c=='D'){
for(i=0;i<n;i++)d[i]=-1;
for(i=0;i<n;i++)if(a[i]!='#')break;
if(i==n){a[0]='D';continue;}
for(;i<n;i++)if(a[i]=='#')d[i]=d[i-1]+1;else d[i]=0;
for(i=0;i<n;i++)e[i]=-1;
for(i=n-1;i>=0;i--)if(a[i]!='#')break;
for(;i>=0;i--)if(a[i]=='#')e[i]=e[i+1]+1;else e[i]=0;
for(i=0;i<n;i++)if(d[i]<e[i])d[i]=e[i];
dd=d[n-1];
for(i=n-1;i>=0;i--)if(dd<=d[i]){dd=d[i];k=i;}
a[k]='D';
}
}
a[n]=0;cout<<a<<endl;
}
return 0;
}

C++: // 2011/07/21 Tazoe

#include <iostream>
using namespace std;

int main()
{
	while(true){
		int n, m;
		cin >> n >> m;
		if(n==0&&m==0)
			break;

		char C[100];
		for(int i=0; i<n; i++)
			C[i] = '#';

		for(int i=0; i<m; i++){
			char a;
			cin >> a;

			bool flg;
			switch(a){
				case 'A':
					for(int j=0; j<n; j++)
						if(C[j]=='#'){
							C[j] = a;
							break;
						}
					break;
				case 'B':
					flg = false;
					for(int j=n-1; j>=0; j--)
						if(C[j]=='#'&&(j==0||C[j-1]!='A')&&(j==n-1||C[j+1]!='A')){
							C[j] = a;
							flg = true;
							break;
						}
					if(!flg)
						for(int j=0; j<n; j++)
							if(C[j]=='#'){
								C[j] = a;
								break;
							}
					break;
				case 'C':
					if(i==0){
						C[n/2] = a;
						break;
					}
					for(int j=0; j<n; j++)
						if(C[j]!='#'&&j!=n-1&&C[j+1]=='#'){
							C[j+1] = a;
							break;
						}
						else if(C[j]!='#'&&j!=0&&C[j-1]=='#'){
							C[j-1] = a;
							break;
						}
					break;
				case 'D':
					if(i==0){
						C[0] = a;
						break;
					}
					int D[100];
					for(int j=0; j<n; j++){
						if(C[j]!='#'){
							D[j] = 0;
							continue;
						}
						int d = 0;
						for(int k=j; k<n; k++, d++)
							if(C[k]!='#')
								break;
						D[j] = d;
						d = 0;
						for(int k=j; k>=0; k--, d++)
							if(C[k]!='#')
								break;
						if(d<D[j])
							D[j] = d;
					}
					int maxD = -1;
					int maxI = -1;
					for(int j=0; j<n; j++)
						if(maxD<D[j]){
							maxD = D[j];
							maxI = j;
						}
					C[maxI] = a;				
					break;
			}
		}

		for(int i=0; i<n; i++)
			cout << C[i];
		cout << endl;
	}

	return 0;
}