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

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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:

1. 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.

2. 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.

3. 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.

5. 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.

6. 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;
}