2707. Extra Characters in a String
Difficulty: Medium
Topics: Array, Hash Table, String, Dynamic Programming, Trie
You are given a 0-indexed string s and a dictionary of words dictionary. You have to break s into one or more non-overlapping substrings such that each substring is present in dictionary. There may be some extra characters in s which are not present in any of the substrings.
Return the minimum number of extra characters left over if you break up s optimally.
Example 1:
Example 2:
Constraints:
Hint:
Solution:
We can define a dp array where dp[i] represents the minimum number of extra characters in the substring s[0:i] after optimal segmentation.
Dynamic Programming Definition:
Transition:
Result:
Let's implement this solution in PHP: 2707. Extra Characters in a String
Explanation:
Base Case:
- dp[0] = 0 since no extra characters exist in an empty string.
Dictionary Lookup:
- We store the dictionary words in a hash map using array_flip() for constant-time lookup.
Transition:
- For each position i, we check all possible substrings s[j:i]. If a substring exists in the dictionary, we update the dp[i] value.
Time Complexity:
- The time complexity is O(n^2) where n is the length of the string s because for each index, we check all previous indices to form substrings.
Test Results:
For the input "leetscode" with dictionary ["leet","code","leetcode"], the function correctly returns 1, as only 1 extra character ("s") remains.
For the input "sayhelloworld" with dictionary ["hello","world"], the function returns 3, as the first three characters ("say") are extra.
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