1466. Reorder Routes to Make All Paths Lead to the City Zero

Description

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] = [a_i, b_i] represents a road from city a_i to city b_i.

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:

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

Example 2:

Input: n = 5, connections = [[1,0],[1,2],[3,2],[3,4]]
Output: 2
Explanation: Change the direction of edges show in red such that each node can reach the node 0 (capital).

Example 3:

Input: n = 3, connections = [[1,0],[2,0]]
Output: 0

Constraints:

2 <= n <= 5 * 10⁴
connections.length == n - 1
connections[i].length == 2
0 <= a_i, b_i <= n - 1
a_i != b_i

Solution

reorder-routes-to-make-all-paths-lead-to-the-city-zero.py

class Solution:
    def minReorder(self, n: int, connections: List[List[int]]) -> int:
        visited = [False] * n
        graph = defaultdict(list)

        for x, y in connections:
            graph[x].append(y)
            graph[y].append(-x)

        def go(node):
            visited[node] = True
            res = 0

            for nei in graph[node]:
                if not visited[abs(nei)]:
                    visited[abs(nei)] = True
                    res += go(abs(nei)) + int(nei > 0)

            return res

        return go(0)