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Deven
39 lines
1.7 KiB
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39 lines
1.7 KiB
Markdown
<h2><a href="https://leetcode.com/problems/count-and-say/">38. Count and Say</a></h2><h3>Medium</h3><hr><div><p>The <strong>count-and-say</strong> sequence is a sequence of digit strings defined by the recursive formula:</p>
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<ul>
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<li><code>countAndSay(1) = "1"</code></li>
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<li><code>countAndSay(n)</code> is the way you would "say" the digit string from <code>countAndSay(n-1)</code>, which is then converted into a different digit string.</li>
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</ul>
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<p>To determine how you "say" a digit string, split it into the <strong>minimal</strong> number of substrings such that each substring contains exactly <strong>one</strong> unique digit. Then for each substring, say the number of digits, then say the digit. Finally, concatenate every said digit.</p>
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<p>For example, the saying and conversion for digit string <code>"3322251"</code>:</p>
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<img alt="" src="https://assets.leetcode.com/uploads/2020/10/23/countandsay.jpg" style="width: 581px; height: 172px;">
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<p>Given a positive integer <code>n</code>, return <em>the </em><code>n<sup>th</sup></code><em> term of the <strong>count-and-say</strong> sequence</em>.</p>
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<p> </p>
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<p><strong>Example 1:</strong></p>
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<pre><strong>Input:</strong> n = 1
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<strong>Output:</strong> "1"
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<strong>Explanation:</strong> This is the base case.
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre><strong>Input:</strong> n = 4
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<strong>Output:</strong> "1211"
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<strong>Explanation:</strong>
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countAndSay(1) = "1"
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countAndSay(2) = say "1" = one 1 = "11"
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countAndSay(3) = say "11" = two 1's = "21"
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countAndSay(4) = say "21" = one 2 + one 1 = "12" + "11" = "1211"
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</pre>
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<p> </p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 <= n <= 30</code></li>
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</ul>
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</div> |