Extra Characters in a String

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:

• Input: s = "leetscode", dictionary = ["leet","code","leetcode"]
• Output: 1
• Explanation: We can break s in two substrings: "leet" from index 0 to 3 and "code" from index 5 to 8. There is only 1 unused character (at index 4), so we return 1.

Example 2:

• Input: s = "sayhelloworld", dictionary = ["hello","world"]
• Output: 3
• Explanation: We can break s in two substrings: "hello" from index 3 to 7 and "world" from index 8 to 12. The characters at indices 0, 1, 2 are not used in any substring and thus are considered as extra characters. Hence, we return 3.

Constraints:

• 1
• 1
• 1
• dictionary[i] and s consists of only lowercase English letters
• dictionary contains distinct words

Hint:

1. Can we use Dynamic Programming here?
2. Define DP[i] as the min extra character if breaking up s[0:i] optimally.

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.

Approach:

1. Dynamic Programming Definition:

   • Let dp[i] be the minimum number of extra characters in the substring s[0:i].
   • To calculate dp[i], we can:
     • Either consider the character s[i-1] as an extra character and move to the next index.
     • Or check if some substring ending at index i exists in the dictionary, and if it does, then use it to reduce extra characters.

2. Transition:

   • For each index i, we either:
     • Add one to dp[i-1] if we treat s[i] as an extra character.
     • Check every possible substring s[j:i] (for j

3. Result:

   • The value of dp[len(s)] will give us the minimum number of extra characters in the entire string s.

Let's implement this solution in PHP: 2707. Extra Characters in a String

<?php /**
 * @param String $s
 * @param String[] $dictionary
 * @return Integer
 */
function minExtraChar($s, $dictionary) {
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
}

// Test cases
echo minExtraChar("leetscode", ["leet","code","leetcode"]); // Output: 1
echo "\n";
echo minExtraChar("sayhelloworld", ["hello","world"]); // Output: 3
?>

Explanation:

1. Base Case:

   • dp[0] = 0 since no extra characters exist in an empty string.

2. Dictionary Lookup:

   • We store the dictionary words in a hash map using array_flip() for constant-time lookup.

3. 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.

4. 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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