383-ransom-note

Question

https://leetcode.com/problems/ransom-note/description/

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.

Example:

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

Thought Process

1. Map
   1. Use separate map to store the frequency of all characters
   2. Any time we run into character that has count == 0, we know we can't construct the note
   3. Time complexity O(n)
   4. Space complexity O(256) or O(1)

Solution

class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
        int[] count = new int[256];
        for (char c : magazine.toCharArray()) {
            count[c]++;
        }
        for (char c : ransomNote.toCharArray()) {
            if (count[c]-- == 0) return false;
        }
        return true;
    }
}

Additional