714321. Add Minimum Characters
Add Minimum Characters
Summary
Given a string str and an integer k, find the minimum characters that need to be added to str so that all substrings of length k+1 contain at least one character from a given set of characters (e.g., lowercase English letters). The problem involves manipulating strings, which is relevant to the "Strings" topic tag.
Detailed Explanation
To solve this problem, we'll create a prefix tree (also known as a trie) to keep track of the characters in str and their positions. We'll then iterate through all substrings of length k+1 and check if they contain at least one character from the given set.
Here's a step-by-step breakdown:
- Create an empty prefix tree.
- Iterate through each character
cinstr. - For each
c, add it to the prefix tree as a new node or update its position if it already exists. - Initialize a variable
min_chars_neededto store the minimum characters needed. - Iterate through all substrings of length
k+1instr. For each substring, check if it contains at least one character from the given set. If not, incrementmin_chars_needed. - Return
min_chars_needed.
Here's an ASCII art diagram illustrating the prefix tree:
+---+
c |
+---+---+
a b c
+-----+-----+
e f g h
| | |
i j k l
Time complexity: O(n), where n is the length of str.
Space complexity: O(m), where m is the size of the given set of characters.
Optimized Solutions
Java
public int addMinimumChars(String str, int k) {
Trie trie = new Trie();
for (int i = 0; i < str.length(); i++) {
trie.insert(str.charAt(i), i);
}
int minCharsNeeded = 0;
for (int i = 0; i <= str.length() - k - 1; i++) {
if (!trie.containsAtLeastOneChar(str.substring(i, i + k + 1))) {
minCharsNeeded++;
}
}
return minCharsNeeded;
}