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花花酱 LeetCode 1898. Maximum Number of Removable Characters

You are given two strings s and p where p is a subsequence of s. You are also given a distinct 0-indexed integer array removable containing a subset of indices of s (s is also 0-indexed).

You want to choose an integer k (0 <= k <= removable.length) such that, after removing k characters from s using the first k indices in removablep is still a subsequence of s. More formally, you will mark the character at s[removable[i]] for each 0 <= i < k, then remove all marked characters and check if p is still a subsequence.

Return the maximum k you can choose such that p is still a subsequence of s after the removals.

subsequence of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.

Example 1:

Input: s = "abcacb", p = "ab", removable = [3,1,0]
Output: 2
Explanation: After removing the characters at indices 3 and 1, "abcacb" becomes "accb".
"ab" is a subsequence of "accb".
If we remove the characters at indices 3, 1, and 0, "abcacb" becomes "ccb", and "ab" is no longer a subsequence.
Hence, the maximum k is 2.

Example 2:

Input: s = "abcbddddd", p = "abcd", removable = [3,2,1,4,5,6]
Output: 1
Explanation: After removing the character at index 3, "abcbddddd" becomes "abcddddd".
"abcd" is a subsequence of "abcddddd".

Example 3:

Input: s = "abcab", p = "abc", removable = [0,1,2,3,4]
Output: 0
Explanation: If you remove the first index in the array removable, "abc" is no longer a subsequence.


  • 1 <= p.length <= s.length <= 105
  • 0 <= removable.length < s.length
  • 0 <= removable[i] < s.length
  • p is a subsequence of s.
  • s and p both consist of lowercase English letters.
  • The elements in removable are distinct.

Solution: Binary Search + Two Pointers

If we don’t remove any thing, p is a subseq of s, as we keep removing, at some point L, p is no longer a subseq of s. e.g [0:True, 1: True, …, L – 1: True, L: False, L+1: False, …, m:False], this array is monotonic. We can use binary search to find the smallest L such that p is no long a subseq of s. Ans = L – 1.

For each guess, we can use two pointers to check whether p is subseq of removed(s) in O(n).

Time complexity: O(nlogn)
Space complexity: O(n)


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