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
}

Python: INF = 10 ** 20

while True:
  n, m = map(int, input().split())
  if not n:
    break

  chairs = list("#" * n)

  def sit(s):
    if s == "A":
      chairs[chairs.index("#")] = "A"
    elif s == "B":
      for i in range(n - 1, -1, -1):
        if chairs[i] == "#" and (i == 0 or chairs[i - 1] != "A") and (i == n - 1 or chairs[i + 1] != "A"):
          chairs[i] = "B"
          break
      else:
        chairs[chairs.index("#")] = "B"

    elif s == "C":
      for i in range(1, n):
        if chairs[i] == "#" and chairs[i - 1] != "#":
          chairs[i] = "C"
          break

    else:
      score = [INF for _ in range(n)]
      dist = INF
      for i in range(n):
        if chairs[i] == "#":
          dist += 1
        else:
          dist = 0
        score[i] = dist

      dist = INF
      for i in range(n - 1, -1, -1):
        if chairs[i] == "#":
          dist += 1
        else:
          dist = 0
        score[i] = min(score[i], dist)
      high_score = max(score)
      chairs[score.index(high_score)] = "D"
  
  first = input()
  if first in ["A", "D"]:
    chairs[0] = first
  elif first == "B":
    chairs[-1] = "B"
  else:
    chairs[n // 2] = "C"
  for _ in range(m - 1):
    sit(input())

  print("".join(chairs))

Python: INF = 10 ** 20

def main():
  while True:
    n, m = map(int, input().split())
    if not n:
      break
  
    chairs = list("#" * n)
  
    def sit(s):
      if s == "A":
        chairs[chairs.index("#")] = "A"
      
      elif s == "B":
        for i in range(n - 1, -1, -1):
          if chairs[i] == "#" and (i == 0 or chairs[i - 1] != "A") and (i == n - 1 or chairs[i + 1] != "A"):
            chairs[i] = "B"
            break
        else:
          chairs[chairs.index("#")] = "B"
  
      elif s == "C":
        for i in range(n):
          if chairs[i] != "#":
            if i + 1 <= n - 1 and chairs[i + 1] == "#":
              chairs[i + 1] = "C"
              break
   
            elif i - 1  >= 0 and chairs[i - 1] == "#":
              chairs[i - 1] = "C"
              break
  
      else:
        score = [INF for _ in range(n)]
        dist = INF
        for i in range(n):
          if chairs[i] == "#":
            dist += 1
          else:
            dist = 0
          score[i] = dist
  
        dist = INF
        for i in range(n - 1, -1, -1):
          if chairs[i] == "#":
            dist += 1
          else:
            dist = 0
          score[i] = min(score[i], dist)
        high_score = max(score)
  
        chairs[score.index(high_score)] = "D"
    
    first = input()
    if first in ["A", "D"]:
      chairs[0] = first
    elif first == "B":
      chairs[-1] = "B"
    else:
      chairs[n // 2] = "C"
    
    for _ in range(m - 1):
      sit(input())
    print("".join(chairs))

main()

Python: def left_empty(bench):
    return bench.index('#') 

def a_sit(bench):
    bench[left_empty(bench)] = 'A'
def right_empty(bench):
    for i in reversed(range(len(bench))):
        if bench[i] == '#':
            yield i

def a_next(bench, i):
    if i + 1 < len(bench) and bench[i + 1] == 'A':
        return True
    if 0 <= i - 1 and bench[i - 1] == 'A':
        return True
    return False
            
def b_sit(bench):
    for i in right_empty(bench):
        if not a_next(bench,i):
            bench[i] = 'B'
            return
    bench[left_empty(bench)] = 'B'
def no_people(bench):
    for bi in bench:
        if bi != '#':
            return False
    return True

def not_empty(bench):
    for i in range(len(bench)):
        if bench[i] != '#':
            yield i

def c_sit(bench):
    if no_people(bench):
        bench[len(bench) // 2] = 'C'
        return
    for i in not_empty(bench):
        if i + 1 < len(bench) and bench[i + 1] == '#':
            bench[i + 1] = 'C'
            return
        if 0 <= i - 1 and bench[i - 1] == '#':
            bench[i - 1] = 'C'
            return
def d_sit(bench):
    if no_people(bench):
        bench[0] = 'D'
        return
    tmp = bench[:]
    
    dist = float('inf')
    for i in range(len(tmp)):
        if tmp[i] == '#':
            dist += 1
        else:
            dist = 0
        tmp[i] = dist
    dist = float('inf')
    for i in reversed(range(len(tmp))):
        if tmp[i] != 0:
            dist += 1
        else:
            dist = 0
        tmp[i] = min(tmp[i], dist)
    bench[tmp.index(max(tmp))] = 'D'

import sys
f = sys.stdin


sit = {'A':a_sit,'B':b_sit,'C':c_sit,'D':d_sit}
while True:
    m, n = map(int, f.readline().split())
    if n == 0:
        break
    bench = ['#'] * m
    for i in range(n):
        sit[f.readline().strip()](bench)
    print(''.join(bench))