Google – Binary Clock


Google – Binary Clock
有一个用binary code来表示的clock, 比如10:15可以表示为1010:1111,每个bit是一个小灯泡。
题目是input一个n,输出所有能用n盏灯表示的时间。
比如n = 0的话,只有零点(00:00)一种可能。
[Solution]
backtracking题.
首先得问清楚是12小时制还是24小时制。如果是24小时制,那么小时部分最多可能有5位(10111 = 23),分钟部分最多可能有6位(1111011 = 59)
所以最多只能有9盏灯亮,也就是n不能大于9. 但是递归的时候可以开一个5 + 6 = 11位的数组,当小时或分钟不合法之后再backtracking就好了。
  public List<String> timesWithNLights(int n) {
    List<String> result = new ArrayList<>();
    if (n < 0 || n > 9) {
      return result;
    }
    if (n == 0) {
      result.add("0:0");
      return result;
    }
 
    int[] clock = new int[11];
    backtracking(clock, n, 0, result);
    return result;
  }
 
  private void backtracking(int[] clock, int n, int pos, List<String> result) {
    if (n == 0) {
      int h = 0;
      int m = 0;
      for (int i = 10; i >= 0; i--) {
        if (i > 4 && clock[i] == 1) {
          m += (1 << (10 - i));
        }
        if (i <= 4 && clock[i] == 1) {
          h += (1 << (4 - i));
        }
      }
      if (h <= 23 && m <= 59) {
        result.add(String.valueOf(h) + ":" + String.valueOf(m));
      }
      return;
    }
 
    int i = pos;
    while (i < clock.length) {
      clock[i] = 1;
      n--;
      backtracking(clock, n, i + 1, result);
      n++;
      clock[i] = 0;
      i++;
    }
  }
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