G - Longest Path https://atcoder.jp/contests/dp/tasks/dp_g

dp_g.py

import sys

sys.setrecursionlimit(10**7)


def dfs(v):
    if dp[v] != -1:             # 調査済み
        return dp[v]
    ret = 0
    for nxt in G[v]:
        ret = max(ret, dfs(nxt) + 1)
    dp[v] = ret
    return ret


N, M = map(int, input().split())
G = [[] * N for _ in range(N)]
for _ in range(M):
    x, y = map(lambda v: int(v) - 1, input().split())
    G[x].append(y)
dp = [-1] * N
ans = 0
for v in range(N):
    ans = max(ans, dfs(v))
print(ans)