{ "data": { "question": { "questionId": "1576", "questionFrontendId": "1466", "categoryTitle": "Algorithms", "boundTopicId": 266094, "title": "Reorder Routes to Make All Paths Lead to the City Zero", "titleSlug": "reorder-routes-to-make-all-paths-lead-to-the-city-zero", "content": "

There are n cities numbered from 0 to n - 1 and n - 1 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.

\n\n

Roads are represented by connections where connections[i] = [ai, bi] represents a road from city ai to city bi.

\n\n

This year, there will be a big event in the capital (city 0), and many people want to travel to this city.

\n\n

Your task consists of reorienting some roads such that each city can visit the city 0. Return the minimum number of edges changed.

\n\n

It's guaranteed that each city can reach city 0 after reorder.

\n\n

 

\n

Example 1:

\n\"\"\n
\nInput: n = 6, connections = [[0,1],[1,3],[2,3],[4,0],[4,5]]\nOutput: 3\nExplanation: Change the direction of edges show in red such that each node can reach the node 0 (capital).\n
\n\n

Example 2:

\n\"\"\n
\nInput: n = 5, connections = [[1,0],[1,2],[3,2],[3,4]]\nOutput: 2\nExplanation: Change the direction of edges show in red such that each node can reach the node 0 (capital).\n
\n\n

Example 3:

\n\n
\nInput: n = 3, connections = [[1,0],[2,0]]\nOutput: 0\n
\n\n

 

\n

Constraints:

\n\n\n", "translatedTitle": "重新规划路线", "translatedContent": "

n 座城市，从 0 到 n-1 编号，其间共有 n-1 条路线。因此，要想在两座不同城市之间旅行只有唯一一条路线可供选择（路线网形成一颗树）。去年，交通运输部决定重新规划路线，以改变交通拥堵的状况。

\n\n

路线用 connections 表示，其中 connections[i] = [a, b] 表示从城市 a 到 b 的一条有向路线。

\n\n

今年，城市 0 将会举办一场大型比赛，很多游客都想前往城市 0 。

\n\n

请你帮助重新规划路线方向，使每个城市都可以访问城市 0 。返回需要变更方向的最小路线数。

\n\n

题目数据 保证 每个城市在重新规划路线方向后都能到达城市 0 。

\n\n

 

\n\n

示例 1：

\n\n

\"\"

\n\n
输入：n = 6, connections = [[0,1],[1,3],[2,3],[4,0],[4,5]]\n输出：3\n解释：更改以红色显示的路线的方向，使每个城市都可以到达城市 0 。
\n\n

示例 2：

\n\n

\"\"

\n\n
输入：n = 5, connections = [[1,0],[1,2],[3,2],[3,4]]\n输出：2\n解释：更改以红色显示的路线的方向，使每个城市都可以到达城市 0 。
\n\n

示例 3：

\n\n
输入：n = 3, connections = [[1,0],[2,0]]\n输出：0\n
\n\n

 

\n\n

提示：

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