leetcode-problemset/leetcode-cn/problem (English)/实现 strStr()(English) [implement-strstr].html

<p>Implement <a href="http://www.cplusplus.com/reference/cstring/strstr/" target="_blank">strStr()</a>.</p>

<p>Given two strings <code>needle</code> and <code>haystack</code>, return the index of the first occurrence of <code>needle</code> in <code>haystack</code>, or <code>-1</code> if <code>needle</code> is not part of <code>haystack</code>.</p>

<p><strong>Clarification:</strong></p>

<p>What should we return when <code>needle</code> is an empty string? This is a great question to ask during an interview.</p>

<p>For the purpose of this problem, we will return 0 when <code>needle</code> is an empty string. This is consistent to C&#39;s <a href="http://www.cplusplus.com/reference/cstring/strstr/" target="_blank">strstr()</a> and Java&#39;s <a href="https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#indexOf(java.lang.String)" target="_blank">indexOf()</a>.</p>

<p>&nbsp;</p>
<p><strong>Example 1:</strong></p>

<pre>
<strong>Input:</strong> haystack = &quot;hello&quot;, needle = &quot;ll&quot;
<strong>Output:</strong> 2
</pre>

<p><strong>Example 2:</strong></p>

<pre>
<strong>Input:</strong> haystack = &quot;aaaaa&quot;, needle = &quot;bba&quot;
<strong>Output:</strong> -1
</pre>

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

<ul>
	<li><code>1 &lt;= haystack.length, needle.length &lt;= 10<sup>4</sup></code></li>
	<li><code>haystack</code> and <code>needle</code> consist of only lowercase English characters.</li>
</ul>