mirror of
https://gitee.com/coder-xiaomo/leetcode-problemset
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188 lines
16 KiB
JSON
188 lines
16 KiB
JSON
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"title": "Count Sorted Vowel Strings",
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"content": "<p>Given an integer <code>n</code>, return <em>the number of strings of length </em><code>n</code><em> that consist only of vowels (</em><code>a</code><em>, </em><code>e</code><em>, </em><code>i</code><em>, </em><code>o</code><em>, </em><code>u</code><em>) and are <strong>lexicographically sorted</strong>.</em></p>\n\n<p>A string <code>s</code> is <strong>lexicographically sorted</strong> if for all valid <code>i</code>, <code>s[i]</code> is the same as or comes before <code>s[i+1]</code> in the alphabet.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> n = 1\n<strong>Output:</strong> 5\n<strong>Explanation:</strong> The 5 sorted strings that consist of vowels only are <code>["a","e","i","o","u"].</code>\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> n = 2\n<strong>Output:</strong> 15\n<strong>Explanation:</strong> The 15 sorted strings that consist of vowels only are\n["aa","ae","ai","ao","au","ee","ei","eo","eu","ii","io","iu","oo","ou","uu"].\nNote that "ea" is not a valid string since 'e' comes after 'a' in the alphabet.\n</pre>\n\n<p><strong class=\"example\">Example 3:</strong></p>\n\n<pre>\n<strong>Input:</strong> n = 33\n<strong>Output:</strong> 66045\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= n <= 50</code> </li>\n</ul>\n",
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"For each character, its possible values will depend on the value of its previous character, because it needs to be not smaller than it.",
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"Think backtracking. Build a recursive function count(n, last_character) that counts the number of valid strings of length n and whose first characters are not less than last_character.",
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"In this recursive function, iterate on the possible characters for the first character, which will be all the vowels not less than last_character, and for each possible value c, increase the answer by count(n-1, c)."
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