mirror of
https://gitee.com/coder-xiaomo/leetcode-problemset
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193 lines
18 KiB
JSON
193 lines
18 KiB
JSON
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"title": "Reorder Routes to Make All Paths Lead to the City Zero",
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"content": "<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>\n\n<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>\n\n<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>\n\n<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>\n\n<p>It's <strong>guaranteed</strong> that each city can reach city <code>0</code> after reorder.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2020/05/13/sample_1_1819.png\" style=\"width: 311px; height: 189px;\" />\n<pre>\n<strong>Input:</strong> n = 6, connections = [[0,1],[1,3],[2,3],[4,0],[4,5]]\n<strong>Output:</strong> 3\n<strong>Explanation: </strong>Change the direction of edges show in red such that each node can reach the node 0 (capital).\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2020/05/13/sample_2_1819.png\" style=\"width: 509px; height: 79px;\" />\n<pre>\n<strong>Input:</strong> n = 5, connections = [[1,0],[1,2],[3,2],[3,4]]\n<strong>Output:</strong> 2\n<strong>Explanation: </strong>Change the direction of edges show in red such that each node can reach the node 0 (capital).\n</pre>\n\n<p><strong class=\"example\">Example 3:</strong></p>\n\n<pre>\n<strong>Input:</strong> n = 3, connections = [[1,0],[2,0]]\n<strong>Output:</strong> 0\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>2 <= n <= 5 * 10<sup>4</sup></code></li>\n\t<li><code>connections.length == n - 1</code></li>\n\t<li><code>connections[i].length == 2</code></li>\n\t<li><code>0 <= a<sub>i</sub>, b<sub>i</sub> <= n - 1</code></li>\n\t<li><code>a<sub>i</sub> != b<sub>i</sub></code></li>\n</ul>\n",
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"Treat the graph as undirected. Start a dfs from the root, if you come across an edge in the forward direction, you need to reverse the edge."
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