Read N Characters Given Read4
<p>Given a <code>file</code> and assume that you can only read the file using a given method <code>read4</code>, implement a method to read <code>n</code> characters.</p>

<p><strong>Method read4: </strong></p>

<p>The API <code>read4</code> reads <strong>four consecutive characters</strong> from <code>file</code>, then writes those characters into the buffer array <code>buf4</code>.</p>

<p>The return value is the number of actual characters read.</p>

<p>Note that <code>read4()</code> has its own file pointer, much like <code>FILE *fp</code> in C.</p>

<p><strong>Definition of read4:</strong></p>

<pre>
    Parameter:  char[] buf4
    Returns:    int

buf4[] is a destination, not a source. The results from read4 will be copied to buf4[].
</pre>

<p>Below is a high-level example of how <code>read4</code> works:</p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0100-0199/0157.Read%20N%20Characters%20Given%20Read4/images/157_example.png" style="width: 600px; height: 403px;" />
<pre>
File file(&quot;abcde<code>&quot;); // File is &quot;</code>abcde<code>&quot;, initially file pointer (fp) points to &#39;a&#39;
char[] buf4 = new char[4]; // Create buffer with enough space to store characters
read4(buf4); // read4 returns 4. Now buf4 = &quot;abcd&quot;, fp points to &#39;e&#39;
read4(buf4); // read4 returns 1. Now buf4 = &quot;e&quot;, fp points to end of file
read4(buf4); // read4 returns 0. Now buf4 = &quot;&quot;, fp points to end of file</code>
</pre>

<p>&nbsp;</p>

<p><strong>Method read:</strong></p>

<p>By using the <code>read4</code> method, implement the method read that reads <code>n</code> characters from <code>file</code> and store it in the buffer array <code>buf</code>. Consider that you cannot manipulate <code>file</code> directly.</p>

<p>The return value is the number of actual characters read.</p>

<p><strong>Definition of read: </strong></p>

<pre>
    Parameters:	char[] buf, int n
    Returns:	int

buf[] is a destination, not a source. You will need to write the results to buf[].
</pre>

<p><strong>Note:</strong></p>

<ul>
	<li>Consider that you cannot manipulate the file directly. The file is only accessible for <code>read4</code> but not for <code>read</code>.</li>
	<li>The <code>read</code> function will only be called once for each test case.</li>
	<li>You may assume the destination buffer array, <code>buf</code>, is guaranteed to have enough space for storing <code>n</code> characters.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> file = &quot;abc&quot;, n = 4
<strong>Output:</strong> 3
<strong>Explanation:</strong> After calling your read method, buf should contain &quot;abc&quot;. We read a total of 3 characters from the file, so return 3.
Note that &quot;abc&quot; is the file&#39;s content, not buf. buf is the destination buffer that you will have to write the results to.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> file = &quot;abcde&quot;, n = 5
<strong>Output:</strong> 5
<strong>Explanation:</strong> After calling your read method, buf should contain &quot;abcde&quot;. We read a total of 5 characters from the file, so return 5.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> file = &quot;abcdABCD1234&quot;, n = 12
<strong>Output:</strong> 12
<strong>Explanation:</strong> After calling your read method, buf should contain &quot;abcdABCD1234&quot;. We read a total of 12 characters from the file, so return 12.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= file.length &lt;= 500</code></li>
	<li><code>file</code> consist of English letters and digits.</li>
	<li><code>1 &lt;= n &lt;= 1000</code></li>
</ul>
Array; Interactive; Simulation
/**
 * The read4 API is defined in the parent class Reader4.
 *     int read4(char[] buf4);
 */

public class Solution extends Reader4 {
    /**
     * @param buf Destination buffer
     * @param n   Number of characters to read
     * @return    The number of actual characters read
     */
    public int read(char[] buf, int n) {
        char[] buf4 = new char[4];
        int i = 0, v = 5;
        while (v >= 4) {
            v = read4(buf4);
            for (int j = 0; j < v; ++j) {
                buf[i++] = buf4[j];
                if (i >= n) {
                    return n;
                }
            }
        }
        return i;
    }
}