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0290 - Word Pattern (Easy)

https://leetcode.com/problems/word-pattern/

Problem Statement​

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.

Approach 1: Hashmap​

Written by @wingkwong
class Solution {
public:
    bool wordPattern(string pattern, string s) {
        // convert s to a vector of strings
        // e.g. "dog cat cat dog" -> ["dog", "cat", "cat", "dog"]
        stringstream ss(s);
        string word;
        vector<string> words;
        while (ss >> word) {
            words.push_back(word);
        }
        // the size of words needs to be same as that of pattern
        // e.g. words = ["xxx"], pattern = "xxx"
        if (words.size() != pattern.size()) {
            return false;
        }
        // for each word in words ...
        // char in pattern -> word
        // e.g. a -> dog
        // e.g. b -> cat
        unordered_map<char, string> m;
        set<string> used;
        for (int i = 0; i < words.size(); i++) {
            // check if map the pattern
            if (m.count(pattern[i])) {
                // if pattern[i] exists in the hashmap,
                // then we need to make sure that the word is correct
                if (m[pattern[i]] != words[i]) {
                    return false;
                }
            } else {
                // each word can only map to one pattern
                // e.g. pattern = "ab", s = "dog dog"
                if (used.find(words[i]) != used.end()) {
                    return false;
                }
                // if not, then map it   
                // e.g. a -> dog
                m[pattern[i]] = words[i];
                used.insert(words[i]);
            }
        }
        return true;
    }
};