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<iostream>
#include<algorithm>
using namespace std;
int main()
{
  while(true){
    int n,m;
    cin >> n >> m;
    if( n == 0 && m == 0 ) break;
    char C[n+2];
    fill(C,C+n+2,'#');
    for(int p = 0; p < m; ++p){
      char c;
      cin >> c;
      if(c=='A'){
	for(int i = 1; i < 1+n; ++i){
	  if( C[i] == '#' ){
	    C[i] = 'A';
	    break;
	  }
	}
      }else if(c=='B'){
	bool sc=false;
	for(int i = n; i >= 1; --i){
	  if( C[i] == '#' && C[i-1] != 'A' && C[i+1] != 'A' ){
	    C[i] = 'B';
	    sc=true;
	    break;
	  }
	}
	if(!sc){
	  for(int i = 1; i < n+1; ++i){
	    if( C[i] == '#' ){
	      C[i] = 'B';
	      break;
	    }
	  }
	}
      }else if(c=='C'){
	bool sc = false;
	for(int i = 1; i < 1+n; ++i){
	  if( C[i-1] != '#' ){
	    if( C[i] != '#' ) continue;
	    else{
	      C[i] = 'C';
	      sc = true;
	      break;
	    }
	  }
	}
	if( !sc ){
	  if( n % 2 == 0 ){
	    C[ n/2 + 1 ] = 'C';
	  }else{
	    C[ (n+1)/2 ] = 'C';
	  }
	}
      }else if(c=='D'){
	int max_dist = 0;
	int nearest_dist[n+2];
	fill(nearest_dist,nearest_dist+n+2,-1);
	bool empty=true;
	for(int i = 1; i < 1+n; ++i){
	  if( C[i] != '#' ) empty=false;
	}
	if(empty){C[1]='D';continue;}
	
	for(int i = 1; i < 1+n; ++i){
	  if( C[i] != '#' ) continue;
	  int min_right = n+3, min_left = n+3;
	  for(int j = i - 1; j >= 0; --j){
	    if( C[j] != '#'){
	      min_left = min( min_left, i - j );
	      break;
	    }
	  }
	  for(int j = i + 1; j < 2+n; ++j){
	    if( C[j] != '#'){
	      min_right = min( min_right,  j - i );
	      break;
	    }
	  }

	  nearest_dist[i] = min( min_right, min_left );
	  max_dist = max( max_dist, nearest_dist[i] );
	}
	for(int i = 1; i < i+n; ++i){
	  //cout << nearest_dist[i] << ' ' << max_dist << endl;
	  if( nearest_dist[i] == max_dist ){
	    C[i] = 'D';
	    break;
	  }
	}
      }
    }
    for(int i = 1; i < 1+n; ++i){
      cout << C[i];
    }
    cout << 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(0<i&&a[i-1]!='A'&&i<n-1&&a[i+1]!='A'){a[i]='B';break;}
else if(0<i&&a[i-1]!='A'){a[i]='B';break;}
else if(i<n-1&&a[i+1]!='A'){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++: #include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(){

	int n,m;
	bool en=false;
	while(cin>>n>>m&&!(n==0&&m==0)){
		if(!en)
			en=true;
		else
			cout<<endl;

		char chair[101];
		char ch;
		fill(chair,chair+101,'#');
		for(int i = 0; i < m; i++){
			cin>>ch;
			if(ch=='A'){
				for(int j = 0; j < n; j++){
					if(chair[j]=='#'){
						chair[j]=ch;
						break;
					}
				}
			}
			else if(ch=='B'){
				bool f=false;
				for(int j = n-1; j >=0; j--){
					if(chair[j]=='#'&&((j-1<0||chair[j-1]!='A')&&(j+1>=n||chair[j+1]!='A'))){
						chair[j]=ch;
						f=true;
						break;
					}
				}
				// ÀêÈ©Á½ê
				if(!f){
					for(int j = 0; j < n; j++){
						if(chair[j]=='#'){
							chair[j]=ch;
							break;
						}
					}
				}
			}
			else if(ch=='C'){
				bool f=false;
				for(int j = 0; j < n; j++){
					if(chair[j]!='#'){
						f=true;
						break;
					}
				}
				if(!f){
					chair[n/2]==ch;
				}
				else{
					for(int j = 0; j < n; j++){
						if(chair[j]!='#'){
							if(j+1<n&&chair[j+1]=='#'){
								chair[j+1]=ch;
								f=true;
								break;
							}
							else if((j-1>=0)&&chair[j-1]=='#'){
								chair[j-1]=ch;
								f=true;
								break;
							}
						}
					}
				}
			}
			else if(ch=='D'){
				int dists[101];
				fill(dists,dists+101,1000000);
				bool fff=false;
				for(int j = 0; j < n; j++){
					if(chair[j]!='#'){
						fff=true;
						break;
					}
				}
				if(fff==false){
					chair[0]=ch;
				}
				else{
					for(int j = 0; j < n; j++){
						if(chair[j]!='#'){
							for(int k = 0; k < n; k++){
								dists[k]=min(dists[k],abs(j-k));
							}
						}
					}
					int minCost=1000000;
					int idx;
					for(int j = 0; j < n; j++){
						if(chair[j]=='#'){
							if(minCost>dists[j]){
								minCost=dists[j];
								idx=j;
							}
						}
					}
					chair[idx]=ch;
				}
			}
		}
		for(int i = 0; i < n; i++){
			cout<<chair[i];
		}
	}

	return 0;
}