383. Ransom Note - https://leetcode.com/problems/ransom-note/

canConstruct.java

class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {

        HashMap<Character,Integer> charMap = new HashMap<Character, Integer>();

        for(char c : magazine.toCharArray()) {

            if(charMap.containsKey(c)) {
                int frequency = charMap.get(c);
                charMap.put(c, frequency + 1);
            }
            else {
                charMap.put(c, 1);
            }
        }

        for (char c: ransomNote.toCharArray()) {

            if(charMap.containsKey(c)) {
                int frequency = charMap.get(c);

                if(frequency <= 0) {
                    return false;
                }

                charMap.put(c, frequency - 1);
            }
            else {
                return false;
            }
        }

        return true;
    }
}