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g2801_2900.s2851_string_transformation.readme.md Maven / Gradle / Ivy

2851\. String Transformation

Hard

You are given two strings `s` and `t` of equal length `n`. You can perform the following operation on the string `s`:

*   Remove a **suffix** of `s` of length `l` where `0 < l < n` and append it at the start of `s`.   
    For example, let `s = 'abcd'` then in one operation you can remove the suffix `'cd'` and append it in front of `s` making `s = 'cdab'`.

You are also given an integer `k`. Return _the number of ways in which_ `s` _can be transformed into_ `t` _in **exactly**_ `k` _operations._

Since the answer can be large, return it **modulo** 109 + 7.

**Example 1:**

**Input:** s = "abcd", t = "cdab", k = 2

**Output:** 2

**Explanation:** 

First way: 

In first operation, choose suffix from index = 3, so resulting s = "dabc". 

In second operation, choose suffix from index = 3, so resulting s = "cdab". 

Second way: 

In first operation, choose suffix from index = 1, so resulting s = "bcda".

In second operation, choose suffix from index = 1, so resulting s = "cdab".

**Example 2:**

**Input:** s = "ababab", t = "ababab", k = 1

**Output:** 2

**Explanation:** 

First way: 

Choose suffix from index = 2, so resulting s = "ababab". 

Second way: 

Choose suffix from index = 4, so resulting s = "ababab".

**Constraints:**

*   2 <= s.length <= 5 * 105
*   1 <= k <= 1015
*   `s.length == t.length`
*   `s` and `t` consist of only lowercase English alphabets.