message: Update robot-return-to-origin.cpp
diff: // Space: O(1)
  class Solution {
  +public:
  + bool judgeCircle(string moves) {
  + unordered_map<char, int> count;
  + for (const auto& m : moves) {
  + ++count[m];
  + }
  + return count['L'] == count['R'] && count['U'] == count['D'];
  + }
  +};
  +
  +// Time: O(n)
  +// Space: O(1)
  +class Solution2 {
  public:
  bool judgeCircle(string moves) {
  auto v = 0, h = 0;
  