Submission #2711395

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Source Code Expand

# seishin.py
import sys
sys.setrecursionlimit(10**6)
N = int(input())
*P, = map(int, input().split())
MOD = 10**9 + 7
from itertools import tee

G = [[] for i in range(N+1)]
for i, p in enumerate(P):
    G[p].append(i+1)
Q = [None]*(N+1)
E = [1, 1, 0, 1]
for i in range(N, -1, -1):
    def gen(i):
        *I, = map(Q.__getitem__, G[i])
        r = E
        while r:
            a,b,c,d=r
            yield (a+c,b,0,d)
            r = None
            for it in I:
                s = next(it, None)
                if s:
                    if r:
                        a0, a1, a2, c0 = s
                        b0, b1, b2, c1 = r
                        r = a0*b0 % MOD, (a0*b1 + a1*b0) % MOD, ((a0 + a1)*b2 + a1*b1) % MOD, c0+c1
                    else:
                        r = s
    Q[i] = gen(i)
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