mirror of
https://gitee.com/coder-xiaomo/leetcode-problemset
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202 lines
20 KiB
JSON
202 lines
20 KiB
JSON
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"title": "Maximum Palindromes After Operations",
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"content": "<p>You are given a <strong>0-indexed</strong> string array <code>words</code> having length <code>n</code> and containing <strong>0-indexed</strong> strings.</p>\n\n<p>You are allowed to perform the following operation <strong>any</strong> number of times (<strong>including</strong> <strong>zero</strong>):</p>\n\n<ul>\n\t<li>Choose integers <code>i</code>, <code>j</code>, <code>x</code>, and <code>y</code> such that <code>0 <= i, j < n</code>, <code>0 <= x < words[i].length</code>, <code>0 <= y < words[j].length</code>, and <strong>swap</strong> the characters <code>words[i][x]</code> and <code>words[j][y]</code>.</li>\n</ul>\n\n<p>Return <em>an integer denoting the <strong>maximum</strong> number of <span data-keyword=\"palindrome-string\">palindromes</span> </em><code>words</code><em> can contain, after performing some operations.</em></p>\n\n<p><strong>Note:</strong> <code>i</code> and <code>j</code> may be equal during an operation.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> words = ["abbb","ba","aa"]\n<strong>Output:</strong> 3\n<strong>Explanation:</strong> In this example, one way to get the maximum number of palindromes is:\nChoose i = 0, j = 1, x = 0, y = 0, so we swap words[0][0] and words[1][0]. words becomes ["bbbb","aa","aa"].\nAll strings in words are now palindromes.\nHence, the maximum number of palindromes achievable is 3.</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> words = ["abc","ab"]\n<strong>Output:</strong> 2\n<strong>Explanation: </strong>In this example, one way to get the maximum number of palindromes is: \nChoose i = 0, j = 1, x = 1, y = 0, so we swap words[0][1] and words[1][0]. words becomes ["aac","bb"].\nChoose i = 0, j = 0, x = 1, y = 2, so we swap words[0][1] and words[0][2]. words becomes ["aca","bb"].\nBoth strings are now palindromes.\nHence, the maximum number of palindromes achievable is 2.\n</pre>\n\n<p><strong class=\"example\">Example 3:</strong></p>\n\n<pre>\n<strong>Input:</strong> words = ["cd","ef","a"]\n<strong>Output:</strong> 1\n<strong>Explanation:</strong> In this example, there is no need to perform any operation.\nThere is one palindrome in words "a".\nIt can be shown that it is not possible to get more than one palindrome after any number of operations.\nHence, the answer is 1.</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= words.length <= 1000</code></li>\n\t<li><code>1 <= words[i].length <= 100</code></li>\n\t<li><code>words[i]</code> consists only of lowercase English letters.</li>\n</ul>\n",
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"We can redistribute all the letters freely among the words.",
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"Calculate the frequency of each letter and total the number of matching letter pairs that can be formed from the letters, i.e., <code>total = sum(freq[ch] / 2)</code> for all <code>'a' <= ch <= 'z'</code>.",
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"We can greedily try making palindromes from <code>words[i]</code> with the smallest length to <code>words[i]</code> with the longest length.",
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"For the current index, <code>i</code>, we try to make <code>words[i]</code> a palindrome. We need <code>len(words[i]) / 2</code> matching character pairs, and the letter in the middle (if it exists) can be freely chosen afterward.",
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"We can check if we have enough pairs for index <code>i</code>; if we do, we increase the number of palindromes we can make and decrease the number of pairs we have. Otherwise, we end the loop at this index.",
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"The answer is the number of palindromes we were able to make in the end."
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