Submission #2711329

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Source Code Expand

# seishin.py
N = int(input())
*P, = map(int, input().split())
MOD = 10**9 + 7

G = [[] for i in range(N+1)]
for i, p in enumerate(P):
    G[p].append(i+1)
Q = [None]*(N+1)
for i in range(N, -1, -1):

    # 子ノードのqueを集める
    R = None
    for j in G[i]:
        S = Q[j]
        if not R:
            R = S
            continue

        # |R| > |S|
        if len(R) < len(S):
            R, S = S, R

        # queの小さい方から大きい方へマージする処理
        for s, r in zip(S, R):
            a0, a1, a2, c0 = s
            b0, b1, b2, c1 = r
            r[:] = a0*b0 % MOD, (a0*b1 + a1*b0) % MOD, (a2*(b0 + b1 + b2) + (a0 + a1)*b2 + a1*b1) % MOD, c0+c1
    if R:
        # a0 <- a2
        for e in R:
            e[0] += e[2]
            e[2] = 0

        Q[i] = [[1, 1, 0, 1]] + R
    else:
        Q[i] = [[1, 1, 0, 1]]
ans = 0
for a0, a1, a2, c in Q[0]:
    ans += pow(2, N+1-c, MOD) * a1 % MOD
print(ans % MOD)

Submission Info

Judge Result