<p>You are given a <strong>0-indexed</strong> string array <code>words</code>.</p> <p>Let's define a <strong>boolean</strong> function <code>isPrefixAndSuffix</code> that takes two strings, <code>str1</code> and <code>str2</code>:</p> <ul> <li><code>isPrefixAndSuffix(str1, str2)</code> returns <code>true</code> if <code>str1</code> is <strong>both</strong> a <span data-keyword="string-prefix">prefix</span> and a <span data-keyword="string-suffix">suffix</span> of <code>str2</code>, and <code>false</code> otherwise.</li> </ul> <p>For example, <code>isPrefixAndSuffix("aba", "ababa")</code> is <code>true</code> because <code>"aba"</code> is a prefix of <code>"ababa"</code> and also a suffix, but <code>isPrefixAndSuffix("abc", "abcd")</code> is <code>false</code>.</p> <p>Return <em>an integer denoting the <strong>number</strong> of index pairs </em><code>(i, j)</code><em> such that </em><code>i < j</code><em>, and </em><code>isPrefixAndSuffix(words[i], words[j])</code><em> is </em><code>true</code><em>.</em></p> <p> </p> <p><strong class="example">Example 1:</strong></p> <pre> <strong>Input:</strong> words = ["a","aba","ababa","aa"] <strong>Output:</strong> 4 <strong>Explanation:</strong> In this example, the counted index pairs are: i = 0 and j = 1 because isPrefixAndSuffix("a", "aba") is true. i = 0 and j = 2 because isPrefixAndSuffix("a", "ababa") is true. i = 0 and j = 3 because isPrefixAndSuffix("a", "aa") is true. i = 1 and j = 2 because isPrefixAndSuffix("aba", "ababa") is true. Therefore, the answer is 4.</pre> <p><strong class="example">Example 2:</strong></p> <pre> <strong>Input:</strong> words = ["pa","papa","ma","mama"] <strong>Output:</strong> 2 <strong>Explanation:</strong> In this example, the counted index pairs are: i = 0 and j = 1 because isPrefixAndSuffix("pa", "papa") is true. i = 2 and j = 3 because isPrefixAndSuffix("ma", "mama") is true. Therefore, the answer is 2. </pre> <p><strong class="example">Example 3:</strong></p> <pre> <strong>Input:</strong> words = ["abab","ab"] <strong>Output:</strong> 0 <strong>Explanation: </strong>In this example, the only valid index pair is i = 0 and j = 1, and isPrefixAndSuffix("abab", "ab") is false. Therefore, the answer is 0.</pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>1 <= words.length <= 50</code></li> <li><code>1 <= words[i].length <= 10</code></li> <li><code>words[i]</code> consists only of lowercase English letters.</li> </ul>