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44 lines
2.7 KiB
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<p>You are given the <code>root</code> of a binary tree with <code>n</code> nodes, where each node is uniquely assigned a value from <code>1</code> to <code>n</code>. You are also given a sequence of <code>n</code> values <code>voyage</code>, which is the <strong>desired</strong> <a href="https://en.wikipedia.org/wiki/Tree_traversal#Pre-order" target="_blank"><strong>pre-order traversal</strong></a> of the binary tree.</p>
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<p>Any node in the binary tree can be <strong>flipped</strong> by swapping its left and right subtrees. For example, flipping node 1 will have the following effect:</p>
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<img alt="" src="https://assets.leetcode.com/uploads/2021/02/15/fliptree.jpg" style="width: 400px; height: 187px;" />
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<p>Flip the <strong>smallest</strong> number of nodes so that the <strong>pre-order traversal</strong> of the tree <strong>matches</strong> <code>voyage</code>.</p>
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<p>Return <em>a list of the values of all <strong>flipped</strong> nodes. You may return the answer in <strong>any order</strong>. If it is <strong>impossible</strong> to flip the nodes in the tree to make the pre-order traversal match </em><code>voyage</code><em>, return the list </em><code>[-1]</code>.</p>
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<p> </p>
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<p><strong>Example 1:</strong></p>
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<img alt="" src="https://assets.leetcode.com/uploads/2019/01/02/1219-01.png" style="width: 150px; height: 205px;" />
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<pre>
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<strong>Input:</strong> root = [1,2], voyage = [2,1]
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<strong>Output:</strong> [-1]
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<strong>Explanation:</strong> It is impossible to flip the nodes such that the pre-order traversal matches voyage.
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</pre>
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<p><strong>Example 2:</strong></p>
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<img alt="" src="https://assets.leetcode.com/uploads/2019/01/02/1219-02.png" style="width: 150px; height: 142px;" />
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<pre>
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<strong>Input:</strong> root = [1,2,3], voyage = [1,3,2]
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<strong>Output:</strong> [1]
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<strong>Explanation:</strong> Flipping node 1 swaps nodes 2 and 3, so the pre-order traversal matches voyage.</pre>
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<p><strong>Example 3:</strong></p>
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<img alt="" src="https://assets.leetcode.com/uploads/2019/01/02/1219-02.png" style="width: 150px; height: 142px;" />
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<pre>
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<strong>Input:</strong> root = [1,2,3], voyage = [1,2,3]
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<strong>Output:</strong> []
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<strong>Explanation:</strong> The tree's pre-order traversal already matches voyage, so no nodes need to be flipped.
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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>The number of nodes in the tree is <code>n</code>.</li>
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<li><code>n == voyage.length</code></li>
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<li><code>1 <= n <= 100</code></li>
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<li><code>1 <= Node.val, voyage[i] <= n</code></li>
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<li>All the values in the tree are <strong>unique</strong>.</li>
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<li>All the values in <code>voyage</code> are <strong>unique</strong>.</li>
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</ul>
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