message: Update reverse-string.cpp
diff: class Solution {
  public:
  - string reverseString(string s) {
  - for (int i = 0, j = s.length() - 1; i < j; ++i, --j) {
  + void reverseString(vector<char>& s) {
  + for (int i = 0, j = s.size() - 1; i < j; ++i, --j) {
  swap(s[i], s[j]);
  }
  - return s;
  }
  };
  @@ -15,8 +14,7 @@ public:
  // Space: O(1)
  class Solution2 {
  public:
  - string reverseString(string s) {
  + void reverseString(vector<char>& s) {
  reverse(s.begin(), s.end());
  - return s;
  }
  };
  