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46 lines
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46 lines
2.0 KiB
HTML
<p>Given an array of integers <code>arr</code>, sort the array by performing a series of <strong>pancake flips</strong>.</p>
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<p>In one pancake flip we do the following steps:</p>
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<ul>
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<li>Choose an integer <code>k</code> where <code>1 <= k <= arr.length</code>.</li>
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<li>Reverse the sub-array <code>arr[0...k-1]</code> (<strong>0-indexed</strong>).</li>
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</ul>
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<p>For example, if <code>arr = [3,2,1,4]</code> and we performed a pancake flip choosing <code>k = 3</code>, we reverse the sub-array <code>[3,2,1]</code>, so <code>arr = [<u>1</u>,<u>2</u>,<u>3</u>,4]</code> after the pancake flip at <code>k = 3</code>.</p>
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<p>Return <em>an array of the </em><code>k</code><em>-values corresponding to a sequence of pancake flips that sort </em><code>arr</code>. Any valid answer that sorts the array within <code>10 * arr.length</code> flips will be judged as correct.</p>
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<p> </p>
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<p><strong class="example">Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> arr = [3,2,4,1]
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<strong>Output:</strong> [4,2,4,3]
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<strong>Explanation: </strong>
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We perform 4 pancake flips, with k values 4, 2, 4, and 3.
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Starting state: arr = [3, 2, 4, 1]
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After 1st flip (k = 4): arr = [<u>1</u>, <u>4</u>, <u>2</u>, <u>3</u>]
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After 2nd flip (k = 2): arr = [<u>4</u>, <u>1</u>, 2, 3]
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After 3rd flip (k = 4): arr = [<u>3</u>, <u>2</u>, <u>1</u>, <u>4</u>]
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After 4th flip (k = 3): arr = [<u>1</u>, <u>2</u>, <u>3</u>, 4], which is sorted.
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</pre>
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<p><strong class="example">Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> arr = [1,2,3]
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<strong>Output:</strong> []
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<strong>Explanation: </strong>The input is already sorted, so there is no need to flip anything.
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Note that other answers, such as [3, 3], would also be accepted.
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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 <= arr.length <= 100</code></li>
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<li><code>1 <= arr[i] <= arr.length</code></li>
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<li>All integers in <code>arr</code> are unique (i.e. <code>arr</code> is a permutation of the integers from <code>1</code> to <code>arr.length</code>).</li>
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</ul>
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