Course Schedule II

Medium

There are n courses labelled 0 to n - 1. You are given edges where edges[i] = [a, b] means you must take course a before course b.

Return the length of a valid ordering in which all courses can be finished, or 0 if no such ordering exists.

The original problem returns the ordering itself. Because many orderings are valid, this judge asks for the length of the order you build — which is n exactly when a full topological sort succeeds, and 0 when a cycle blocks it. Build the real array, then return its length.

Example 1

Input:  n = 4, edges = [[1,0],[2,0],[3,1],[3,2]]
Output: 4
Explanation: One valid order is [3,1,2,0], so the length is 4.

Example 2

Input:  n = 2, edges = [[0,1],[1,0]]
Output: 0
Explanation: The two courses depend on each other, so no ordering exists.

Constraints

• 1 <= n <= 2000
• 0 <= edges.length <= 5000
• edges[i].length == 2
• All pairs are distinct