Strobogrammatic Number
<p>Given a string <code>num</code> which represents an integer, return <code>true</code> <em>if</em> <code>num</code> <em>is a <strong>strobogrammatic number</strong></em>.</p>

<p>A <strong>strobogrammatic number</strong> is a number that looks the same when rotated <code>180</code> degrees (looked at upside down).</p>

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

<pre>
<strong>Input:</strong> num = &quot;69&quot;
<strong>Output:</strong> true
</pre>

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

<pre>
<strong>Input:</strong> num = &quot;88&quot;
<strong>Output:</strong> true
</pre>

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

<pre>
<strong>Input:</strong> num = &quot;962&quot;
<strong>Output:</strong> false
</pre>

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

<ul>
	<li><code>1 &lt;= num.length &lt;= 50</code></li>
	<li><code>num</code> consists of only digits.</li>
	<li><code>num</code> does not contain any leading zeros except for zero itself.</li>
</ul>
Hash Table; Two Pointers; String
class Solution {
    public boolean isStrobogrammatic(String num) {
        int[] d = new int[] {0, 1, -1, -1, -1, -1, 9, -1, 8, 6};
        for (int i = 0, j = num.length() - 1; i <= j; ++i, --j) {
            int a = num.charAt(i) - '0', b = num.charAt(j) - '0';
            if (d[a] != b) {
                return false;
            }
        }
        return true;
    }
}