#2851
String Transformation
hard · failed · 26.8% accepted · 183 likes · top 5%
math · string · dynamic programming · string matching
Description
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".
Example 2:
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 3:
Input: s = "ababab", t = "ababab", k = 1
Output: 2
Explanation:
First way:
Choose suffix from index = 2, so resulting s = "ababab".
Example 4:
Second way:
Choose suffix from index = 4, so resulting s = "ababab".
Solution