位图

详解过程参考https://mp.weixin.qq.com/s/xxauNrJY9HlVNvLrL5j2hg

实现代码：

public class BitMap { // Java中char类型占16bit，也即是2个字节
  private char[] bytes;
  private int nbits;
  
  public BitMap(int nbits) {
    this.nbits = nbits;
    this.bytes = new char[nbits/16+1];
  }

  public void set(int k) {
    if (k > nbits) return;
    int byteIndex = k / 16; //这就是图解算法里说的word
    int bitIndex = k % 16;  //这里是偏移量  
    bytes[byteIndex] |= (1 << bitIndex);
  }

  public boolean get(int k) {
    if (k > nbits) return false;
    int byteIndex = k / 16;
    int bitIndex = k % 16;
    return (bytes[byteIndex] & (1 << bitIndex)) != 0;
  }
}