# Halo

A magic place for coding

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## Problem

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.

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

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

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

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

Example 1:

Example 2:

Example 3:

Constraints:

• 2 <= n <= 5 * 104
• connections.length == n - 1
• connections[i].length == 2
• 0 <= ai, bi <= n - 1
• ai != bi

## Analysis

题目以边的方式给出了一个有向图，要求通过更改某些边的方向，使得所有城市都能到达city0，且更改的数量是最少的。首先因为要更改的数量是最少的，所以当然就是要在最短路径上找了，在一副无向图中找每个点到city0的最短路径这个不难，一个BFS搞定。问题是怎么在有向图中找并且要计算翻转边的数量。首先我们可以把所有的边都看作是双向的，但是用正负值区分开方向，比如graph[i] = {j}大于0说明有一条i指向j的边，如果graph[i] = {j}小于0说明有一条j指向i的边，如果是0就是不存在边了。

然后我们开始遍历，如果不为0说明是可以连接的，如果是正数说明这条边需要被翻转，因为我们是从city0出发，而题目要求的是其他城市到city0，方向刚好相反。我们只需要记录正数边的数量即可。

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## Summary

这道题目的本质还是图的BFS，只不过在方向上加了点难度，通过正负值区分方向是一种很常见的手段。这道题目的分享到这里，感谢你的支持！

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