#571 · #1466 · Reorder Routes to Make All Paths Lead to the City Zero(重新规划路线)

There are n cities numbered 0..n-1 and n - 1 directed roads forming a tree. connections[i] = [a, b] means the road goes from a to b. Reorient some roads so that every city can reach city 0. Return the minimum number of roads to reorient. Constraints: 2 <= n <= 5 * 10^4, connections.length == n - 1.