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# difference use between ArrayList and HashMap in Java

An example: 

When you want to iterate every element in ArrayList:

```java
ArrayList<Character> Mylist = new ArrayList<Character>();
        for (char element : magazine.toCharArray()) {
            Mylist.add(element);
        }
```

When you want to iterate every element in HashMap:

```java
HashMap<Character, Integer> charFreq = new HashMap<>();
    for (char c : magazine.toCharArray()) {
        charFreq.put(c, charFreq.getOrDefault(c, 0) + 1);
    }
```

**Example 1:**

<pre><code><strong>Input: ransomNote = "a", magazine = "b"
</strong><strong>Output: false
</strong></code></pre>

**Example 2:**

<pre><code><strong>Input: ransomNote = "aa", magazine = "ab"
</strong><strong>Output: false
</strong></code></pre>

**Example 3:**

<pre><code><strong>Input: ransomNote = "aa", magazine = "aab"
</strong><strong>Output: true
</strong></code></pre>

Option 1: Use ArrayList:

```java
class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
        if (ransomNote.length() > magazine.length()) {
            return false;
        }
        ArrayList<Character> Mylist = new ArrayList<Character>();
        for (char element : magazine.toCharArray()) {
            Mylist.add(element);
        }
        for (char i : ransomNote.toCharArray()) {
            if (Mylist.contains(i)) {
                Mylist.remove((Character) i);    
            } else {
                return false;
            }
        }
        return true;
    }
} 
```

Option2: Use HashMap:

```java
public boolean canConstruct(String ransomNote, String magazine) {
    if (ransomNote.length() > magazine.length()) {
        return false;
    }
    HashMap<Character, Integer> charFreq = new HashMap<>();
    for (char c : magazine.toCharArray()) {
        charFreq.put(c, charFreq.getOrDefault(c, 0) + 1);
    }
    for (char c : ransomNote.toCharArray()) {
        if (charFreq.containsKey(c) && charFreq.get(c) > 0) {
            charFreq.put(c, charFreq.get(c) - 1);
        } else {
            return false;
        }
    }
    return true;
}
```

In the context of the given code, `char c` is a variable that represents a single character in a string. The variable `c` is used to iterate through each character in the `ransomNote` and `magazine` strings.

The `toCharArray()` method is a built-in function in Java that is used to convert a string to a character array. When this method is called on a string, it returns a newly formed character array with a length equal to the given string and with the characters in the given string initialized as its contents. The returned array length is equal to the length of the string. This method is an instance method of the String class and does not accept any parameter. It is useful when we want to convert a string to a character array without writing any custom code. The `toCharArray()` method is used in the given code to convert the `magazine` and `ransomNote` strings to character arrays so that we can iterate through each character in the strings using a for-each loop.