docs: private int intersectionSize(int[] px, int[] py) {
      int i = 0, j = 0, c = 0;
      while(i < px.length && j < py.length) {
        final int vx = px[i], vy = py[j];
        if(vx < vy) {
          ++i;
        }
        else if(vx > vy) {
          ++j;
        }
        else {
          ++c;
          ++i;
          ++j;
        }
      }
      return c;
    }