Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

memory = {}


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))

    dp = [0]*N
    last = [-1]*(2*N+1)
    for i, s in enumerate(C):
        if i == 0:
            dp[i] = 1
        else:
            dp[i] = dp[i-1]
            if last[s] != -1 and last[s] < i-1:
                dp[i] = (dp[i] + dp[last[s]]) % MOD
        last[s] = i
    # print(dp, souwa, pre)
    print(dp[-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

memory = {}


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))

    dp = [0]*N
    last = [-1]*(2*(10**5)+1)
    for i, s in enumerate(C):
        if i == 0:
            dp[i] = 1
        else:
            dp[i] = dp[i-1]
            if last[s] != -1 and last[s] < i-1:
                dp[i] = (dp[i] + dp[last[s]]) % MOD
        last[s] = i
    # print(dp, souwa, pre)
    print(dp[-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

memory = {}


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))

    dp = [0]*N
    last = [-1]*2*N
    for i, s in enumerate(C):
        if i == 0:
            dp[i] = 1
        else:
            dp[i] = dp[i-1]
            if last[s] != -1 and last[s] < i-1:
                dp[i] = (dp[i] + dp[last[s]]) % MOD
        last[s] = i
    # print(dp, souwa, pre)
    print(dp[-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

memory = {}


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))

    dp = [0]*N
    last = [-1]*(2*(10**5)+1)
    for i, s in enumerate(C):
        if i == 0:
            dp[i] = 1
        else:
            dp[i] = dp[i-1]
            if last[s] != -1 and last[s] < i-1:
                dp[i] = (dp[i] + dp[last[s]]) % MOD
        last[s] = i
    # print(dp, souwa, pre)
    print(dp[-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

memory = {}


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))

    dp = [0]*N
    last = [-1]*N
    for i, s in enumerate(C):
        if i == 0:
            dp[i] = 1
        else:
            dp[i] = dp[i-1]
            if last[s] != -1 and last[s] < i-1:
                dp[i] = (dp[i] + dp[last[s]]) % MOD
        last[s] = i
    # print(dp, souwa, pre)
    print(dp[-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

memory = {}


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))

    dp = [0]*N
    last = [-1]*(2*(10**5)+1)
    for i, s in enumerate(C):
        if i == 0:
            dp[i] = 1
        else:
            dp[i] = dp[i-1]
            if last[s] != -1 and last[s] < i-1:
                dp[i] = (dp[i] + dp[last[s]]) % MOD
        last[s] = i
    # print(dp, souwa, pre)
    print(dp[-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

memory = {}


def dfs(stones, N, depth):
    if N <= 2:
        # print(" "*depth, stones, "leaf")
        return 1
    # print(" "*depth, stones)
    if stones in memory:
        return memory[stones]

    # ans1 はじめの石使わない
    # print(" "*depth, "ans1")
    ans1 = dfs(stones[1:], N-1, depth+1)

    # ans2 はじめの石を使う
    try:
        ind = stones[2:].index(stones[0])
        # print(" "*depth, "ans2", c)
        # print(" "*depth, ind, stones, stones[ind:])
        ans2 = dfs(stones[2+ind:], N-ind-2, depth+1)
    except:
        ans2 = 0

    ans = (ans1 + ans2) % MOD
    memory[stones] = ans
    return ans


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))

    dp = [0]*N
    last = [-1]*N
    for i, s in enumerate(C):
        if i == 0:
            dp[i] = 1
        else:
            dp[i] = dp[i-1]
            if last[s] != -1 and last[s] < i-1:
                dp[i] += dp[last[s]]
        last[s] = i
    # print(dp, souwa, pre)
    print(dp[-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

memory = {}


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))

    dp = [0]*N
    last = [-1]*(2*(10**5)+1)
    for i, s in enumerate(C):
        if i == 0:
            dp[i] = 1
        else:
            dp[i] = dp[i-1]
            if last[s] != -1 and last[s] < i-1:
                dp[i] = (dp[i] + dp[last[s]]) % MOD
        last[s] = i
    # print(dp, souwa, pre)
    print(dp[-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

# memory = {}

# def dfs(stones, N, depth):
#     if N <= 2:
#         # print(" "*depth, stones, "leaf")
#         return 1
#     # print(" "*depth, stones)
#     if stones in memory:
#         return memory[stones]

#     # ans1 はじめの石使わない
#     # print(" "*depth, "ans1")
#     ans1 = dfs(stones[1:], N-1, depth+1)

#     # ans2 はじめの石を使う
#     try:
#         ind = stones[2:].index(stones[0])
#         # print(" "*depth, "ans2", c)
#         # print(" "*depth, ind, stones, stones[ind:])
#         ans2 = dfs(stones[2+ind:], N-ind-2, depth+1)
#     except:
#         ans2 = 0

#     ans = (ans1 + ans2) % MOD
#     memory[stones] = ans
#     return ans


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))
    dp = [0]*N
    pre = 0
    souwa = [0]*N
    for i, s in enumerate(C):
        # print(dp, souwa, pre)
        if i == 0:
            dp[i] = 1
        else:
            if s == pre:
                dp[i] = dp[i-1]
            else:
                souwa[pre-1] = dp[i-1]
                dp[i] = dp[i-1] + souwa[s-1]
        pre = s
    # print(dp, souwa, pre)
    print(dp[N-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int

memory = {}


def solve(N: int, C: "List[int]"):
    # print(dfs(tuple(C), N, 0))

    dp = [0]*N
    last = [-1]*(2*(10**5)+1)
    for i, s in enumerate(C):
        if i == 0:
            dp[i] = 1
        else:
            dp[i] = dp[i-1]
            if last[s] != -1 and last[s] < i-1:
                dp[i] = (dp[i] + dp[last[s]]) % MOD
        last[s] = i
    # print(dp, souwa, pre)
    print(dp[-1])
    return


# Generated by 1.1.3 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    C = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, C)


if __name__ == '__main__':
    main()