383. Ransom Note

O(m+n) time

class Solution {
public:
    bool canConstruct(string ransomNote, string magazine) {
        int freq[26] = {0};
        for (char c : magazine) {
            ++freq[c - 'a'];
        }
        for (char c : ransomNote) {
            if (--freq[c - 'a'] < 0) return false;
        }
        return true;
    }
};