771. Jewels and Stones

• Time: $O(|\texttt{J}| + |\texttt{S}|)$
• Space: $O(|\texttt{J}|)$

C++JavaPython

class Solution {
 public:
  int numJewelsInStones(string J, string S) {
    int ans = 0;
    unordered_set<char> jewels(begin(J), end(J));

    for (const char s : S)
      if (jewels.count(s))
        ++ans;

    return ans;
  }
};

class Solution {
  public int numJewelsInStones(String J, String S) {
    int ans = 0;
    Set<Character> jewels = new HashSet<>();

    for (char j : J.toCharArray())
      jewels.add(j);

    for (final char s : S.toCharArray())
      if (jewels.contains(s))
        ++ans;

    return ans;
  }
}

class Solution:
  def numJewelsInStones(self, J: str, S: str) -> int:
    jewels = set(J)
    return sum(s in jewels for s in S)