message: Update reach-a-number.cpp
diff: -// Time: O(sqrt(n))
  +// Time: O(logn)
  // Space: O(1)
  class Solution {
  +public:
  + int reachNumber(int target) {
  + target = abs(target);
  + auto k = static_cast<int>(ceil((-1 + sqrt(1 + 8.0 * target)) / 2));
  + target -= k * (k + 1) / 2;
  + return (target % 2 == 0) ? k : k + 1 + k % 2;
  + }
  +};
  +
  +
  +// Time: O(sqrt(n))
  +// Space: O(1)
  +class Solution2 {
  public:
  int reachNumber(int target) {
  target = abs(target);
  