#1830
Minimum Number of Operations to Make String Sorted
hard · verified · 50.8% accepted · 188 likes · top 39%
hash table · math · string · combinatorics · counting
Description
You are given a string s (0-indexed). You are asked to perform the following operation on s until you get a sorted string:
- Find the largest index i such that 1 <= i < s.length and s[i] < s[i - 1].
- Find the largest index j such that i <= j < s.length and s[k] < s[i - 1] for all the possible values of k in the range [i, j] inclusive.
- Swap the two characters at indices i - 1 and j.
- Reverse the suffix starting at index i.
Return the number of operations needed to make the string sorted. Since the answer can be too large, return it modulo 109 + 7.
Example 1:
Input: s = "cba"
Output: 5
Explanation: The simulation goes as follows:
Operation 1: i=2, j=2. Swap s[1] and s[2] to get s="cab", then reverse the suffix starting at 2. Now, s="cab".
Operation 2: i=1, j=2. Swap s[0] and s[2] to get s="bac", then reverse the suffix starting at 1. Now, s="bca".
Operation 3: i=2, j=2. Swap s[1] and s[2] to get s="bac", then reverse the suffix starting at 2. Now, s="bac".
Operation 4: i=1, j=1. Swap s[0] and s[1] to get s="abc", then reverse the suffix starting at 1. Now, s="acb".
Operation 5: i=2, j=2. Swap s[1] and s[2] to get s="abc", then reverse the suffix starting at 2. Now, s="abc".
Example 2:
Input: s = "aabaa"
Output: 2
Explanation: The simulation goes as follows:
Operation 1: i=3, j=4. Swap s[2] and s[4] to get s="aaaab", then reverse the substring starting at 3. Now, s="aaaba".
Operation 2: i=4, j=4. Swap s[3] and s[4] to get s="aaaab", then reverse the substring starting at 4. Now, s="aaaab".
Solution