Reorder Routes to Make All Paths Lead to the City Zero

Statement

There are n cities labeled from $0$ to $n - 1$, connected by $n - 1$ roads, so there is only one route between any two cities. This road network forms a tree structure.

Last year, the Ministry of transport made all roads one-way due to their narrow width. These roads are represented as connections, where each entry connections[i] $= [a_i, b_i]$ means there is a road going from city $a_i$ to city $b_i$​.

This year, a major event will occur in the capital city (city $0$), and people from all other cities must reach it. Your task is to reorient some roads so every city has a valid path leading to city $0$. Return the minimum number of roads that must be changed to achieve this.

Note: A solution is guaranteed to exist, meaning that every city can eventually reach city $0$ after reordering.

Constraints:

• $2 \leq$ n $\leq 5 * 10^4$

• connections.length $== n -1$

• connections[i].length $== 2$

• $0 \leq a_i, b_i \leq n - 1$

Examples