047 - Monochromatic Diagonal（★7） https://atcoder.jp/contests/typical90/tasks/typical90_au

typical90_au.py

N = int(input())
S = input()
T = input()
MOD = 699999953
seq1 = [0] * N
seq3 = [0] * N
for i in range(N):
    if S[i] == "G":
        seq1[i] = 1
    elif S[i] == "B":
        seq1[i] = 2
    if T[i] == "G":
        seq3[i] = 1
    elif T[i] == "B":
        seq3[i] = 2

ans = 0
for i in range(3):
    seq2 = [0] * N
    for j in range(N):
        seq2[j] = (i - seq3[j] + 3) % 3
    power3, hash1, hash2 = 1, 0, 0
    for j in range(N):
        hash1 = (hash1 * 3 + seq1[j]) % MOD
        hash2 = (hash2 + power3 * seq2[N - j - 1]) % MOD
        if hash1 == hash2:
            ans += 1
        power3 = power3 * 3 % MOD
    power3, hash1, hash2 = 1, 0, 0
    for j in range(N - 1):
        hash1 = (hash1 + power3 * seq1[N - j - 1]) % MOD
        hash2 = (hash2 * 3 + seq2[j]) % MOD
        if hash1 == hash2:
            ans += 1
        power3 = power3 * 3 % MOD

print(ans)