383. 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.

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

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

Solution

HashTable, O(n + m), S(n)

class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
        if (ransomNote == null || magazine == null 
            || ransomNote.length() > magazine.length()) {
            return false;
        }
        
        int[] count = new int[26];
        
        for (int i = 0; i < magazine.length(); ++i) {
            ++count[magazine.charAt(i) - 'a'];
        }
        
        for (int i = 0; i < ransomNote.length(); ++i) {
            if (--count[ransomNote.charAt(i) - 'a'] < 0) {
                return false;
            }
        }
        
        return true;
    }
}