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45 lines
2.2 KiB
HTML
45 lines
2.2 KiB
HTML
<p>There are <code>n</code> cities numbered from <code>0</code> to <code>n - 1</code> and <code>n - 1</code> roads such that there is only one way to travel between two different cities (this network form a tree). Last year, The ministry of transport decided to orient the roads in one direction because they are too narrow.</p>
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<p>Roads are represented by <code>connections</code> where <code>connections[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> represents a road from city <code>a<sub>i</sub></code> to city <code>b<sub>i</sub></code>.</p>
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<p>This year, there will be a big event in the capital (city <code>0</code>), and many people want to travel to this city.</p>
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<p>Your task consists of reorienting some roads such that each city can visit the city <code>0</code>. Return the <strong>minimum</strong> number of edges changed.</p>
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<p>It's <strong>guaranteed</strong> that each city can reach city <code>0</code> after reorder.</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/2020/05/13/sample_1_1819.png" style="width: 311px; height: 189px;" />
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<pre>
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<strong>Input:</strong> n = 6, connections = [[0,1],[1,3],[2,3],[4,0],[4,5]]
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<strong>Output:</strong> 3
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<strong>Explanation: </strong>Change the direction of edges show in red such that each node can reach the node 0 (capital).
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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/2020/05/13/sample_2_1819.png" style="width: 509px; height: 79px;" />
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<pre>
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<strong>Input:</strong> n = 5, connections = [[1,0],[1,2],[3,2],[3,4]]
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<strong>Output:</strong> 2
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<strong>Explanation: </strong>Change the direction of edges show in red such that each node can reach the node 0 (capital).
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> n = 3, connections = [[1,0],[2,0]]
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<strong>Output:</strong> 0
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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>2 <= n <= 5 * 10<sup>4</sup></code></li>
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<li><code>connections.length == n - 1</code></li>
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<li><code>connections[i].length == 2</code></li>
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<li><code>0 <= a<sub>i</sub>, b<sub>i</sub> <= n - 1</code></li>
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<li><code>a<sub>i</sub> != b<sub>i</sub></code></li>
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</ul>
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