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Process in topological order
Finding the longest path in a general graph is NP-hard.
But in a DAG, it is easy with DP. Idea: Process nodes in topological order. For each node , dp[v] stores the longest path ending at . For each edge , update dp[v] = max(dp[v], dp[u] + 1). Start with dp[source] = 0, all others at . Answer is dp[destination]. Shortest path works the same way, use instead of .
