383. Ransom Note

Given an arbitrary ransom note string and another string containing letters from all the magazines, write a function that will return true if the ransom note can be constructed from the magazines ; otherwise, it will return false.

Each letter in the magazine string can only be used once in your ransom note.

Note: You may assume that both strings contain only lowercase letters.

canConstruct("a", "b") -> false
canConstruct("aa", "ab") -> false
canConstruct("aa", "aab") -> true

Solution

Main idea for this problem is by providing a HashMap to record the character & its count in magazine.

And iterate the ransonNote to find out if there is char not able to be constructed by the HashMap.

public boolean canConstruct(String ransomNote, String magazine) {
        HashMap<Character, Integer> map = new HashMap<>();
        for(int i = 0; i < magazine.length(); i++){
            char c = magazine.charAt(i);
            map.put(c, map.getOrDefault(c, 0) + 1);
        }
        for(int i = 0; i < ransomNote.length(); i++){
            char c = ransomNote.charAt(i);
            if(map.getOrDefault(c, 0) == 0)
                return false;
            else
               map.put(c, map.get(c) - 1);
        }
        return true;
    }