Number of Strings Which Can Be Rearranged to Contain Substring — LeetCode 2930 Python Solution

The problem

You are given an integer n. A string s is called good if it contains only lowercase English characters and it is possible to rearrange the characters of s such that the new string contains "leet" as a substring.

Example

Input

n = 4

Output

12

Explanation

The 12 strings which can be rearranged to have "leet" as a substring are: "eelt", "eetl", "elet", "elte", "etel", "etle", "leet", "lete", "ltee", "teel", "tele", and "tlee".

Python solution

class Solution:
    def stringCount(self, n: int) -> int:
        @cache
        def dfs(i: int, l: int, e: int, t: int) -> int:
            if i == 0:
                return int(l == 1 and e == 2 and t == 1)
            a = dfs(i - 1, l, e, t) * 23 % mod
            b = dfs(i - 1, min(1, l + 1), e, t)
            c = dfs(i - 1, l, min(2, e + 1), t)
            d = dfs(i - 1, l, e, min(1, t + 1))
            return (a + b + c + d) % mod

        mod = 10**9 + 7
        return dfs(n, 0, 0, 0)

Complexity

Pattern: Dynamic Programming

Define a state, write the transition, and stop recomputing the same subproblem. LeetCode 2930. Number of Strings Which Can Be Rearranged to Contain Substring is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Dynamic Programming.

The dynamic programming guide has the Python template for the pattern and the 481 LeetCode problems that use it.

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