LeetCode //C - 290. Word Pattern
290. Word Pattern
Given a pattern and a string s, find if s follows the same pattern.
Here follow means a full match, such that there is a bijection between a letter in pattern and a non-empty word in s.
Example 1:
Input: pattern = “abba”, s = “dog cat cat dog”
Output: true
Example 2:
Input: pattern = “abba”, s = “dog cat cat fish”
Output: false
Example 3:
Input: pattern = “aaaa”, s = “dog cat cat dog”
Output: false
Constraints:
- 1 <= pattern.length <= 300
- pattern contains only lower-case English letters.
- 1 <= s.length <= 3000
- s contains only lowercase English letters and spaces ’ '.
- s does not contain any leading or trailing spaces.
- All the words in s are separated by a single space.
From: LeetCode
Link: 290. Word Pattern
Solution:
Ideas:
The code for the wordPattern function is designed to determine whether a given string s follows a specific pattern. Here’s a breakdown of the idea behind the code:
-
Initialization: The code initializes an array of pointers words, where each index corresponds to a lowercase letter in the pattern (from ‘a’ to ‘z’). This array will be used to track the mapping between each character in the pattern and a corresponding word in the string s.
-
String Copy: Since the code utilizes strtok to tokenize the string s, which alters the original string, a copy of the string is made to preserve the input.
-
Tokenization and Mapping: The code tokenizes the string s into words and iterates through these words alongside the characters in the pattern:
- If the pattern is exhausted before all words are processed, the function returns false, as there is a mismatch in length.
- For each character in the pattern, the code checks whether it has been mapped to a word before. If not, it verifies that no other character has been mapped to the current word (to ensure a bijection). If all is well, the current character is mapped to the current word.
- If the character has already been mapped to a word, the code checks that this word matches the current word in the string. If there’s a mismatch, the function returns false.
-
Length Check: After processing all words, the code checks whether the pattern’s length matches the number of words. If there’s a mismatch, the function returns false.
-
Result: If all checks pass, the function returns true, indicating that the string s follows the given pattern.
Code:
bool wordPattern(char * pattern, char * s) {
char *words[26];
for (int i = 0; i < 26; i ) words[i] = NULL;
char *s_copy = strdup(s);
char *token = strtok(s_copy, " ");
int i = 0;
while (token != NULL) {
// If the pattern is shorter than the number of words, return false
if (i >= strlen(pattern)) {
free(s_copy);
return false;
}
int idx = pattern[i] - 'a';
if (words[idx] == NULL) {
// Check if any other character maps to the same word
for (int j = 0; j < 26; j ) {
if (words[j] && strcmp(words[j], token) == 0) {
free(s_copy);
return false;
}
}
words[idx] = token;
} else {
if (strcmp(words[idx], token) != 0) {
free(s_copy);
return false; // Mismatch in mapping
}
}
token = strtok(NULL, " ");
i ;
}
free(s_copy);
// Check if pattern length matches the number of words
if (i != strlen(pattern)) return false;
return true;
}
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