A very typical application of hash maps. Since I am now learning Java, I code in Java. The following code uses toCharArray() and getOrDefault(), which are learnt from .

public class Solution {    public boolean canConstruct(String ransomNote, String magazine) {        HashMap
     
       map = new HashMap<>();        for (Character k : magazine.toCharArray()) {            int v = map.getOrDefault(k, 0) + 1;            map.put(k, v);        }        for (Character k : ransomNote.toCharArray()) {            int v = map.getOrDefault(k, 0) - 1;            if (v < 0) return false;            map.put(k, v);        }        return true;    }}
     

This code takes about 60ms.

In fact, arrays can be used to replace hash maps since the test cases of this problem only contan the 26 English letters. A array-version solution can be found and is rewritten below.

public class Solution {    public boolean canConstruct(String ransomNote, String magazine) {        int[] map = new int[26];        for (char k : magazine.toCharArray())            map[k - 'a']++;        for (char k : ransomNote.toCharArray()) {            int v = --map[k - 'a'];            if (v < 0) return false;        }        return true;    }}

This version is much faster, about 13ms.