771. Jewels and Stones

1. Description

You’re given strings J representing the types of stones that are jewels, and S representing the stones you have. Each character in S is a type of stone you have. You want to know how many of the stones you have are also jewels.
The letters in J are guaranteed distinct, and all characters in J and S are letters. Letters are case sensitive, so “a” is considered a different type of stone from “A”.

2. Example

Example 1

Input: J = “aA”, S = “aAAbbbb”
Output: 3

Example 2

Input: J = “z”, S = “ZZ”
Output: 0

3. Note

• S and J will consist of letters and have length at most 50.
• The characters in J are distinct.

4. Solutions

Counting Array

m = i_jewels.size(), n = i_stones.size()
Time complexity: O(m + n)
Space complexity: O(m)

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class Solution
{
public:
    // int numJewelsInStones(string J, string S)
    int numJewelsInStones(string &i_jewels, string &i_stones)
    {
        vector<int> isJewel(128, 0);
        for(int i = 0; i < i_jewels.size(); i++)
            isJewel[i_jewels[i]]++;

        int result = 0;
        for(int i = 0; i < i_stones.size(); i++)
            result += (isJewel[i_stones[i]] ? 1 : 0);

        return result;
    }
};