0290. 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

Example 4:

Input: pattern = "abba", s = "dog dog dog dog"
Output: false

Constraints:

• 1 <= pattern.length <= 300
• pattern contains only lower-case English letters.
• 1 <= s.length <= 3000
• s contains only lower-case English letters and spaces ' '.
• s does not contain any leading or trailing spaces.
• All the words in s are separated by a single space.

Analysis

Valid matching:

Invalid matching 1:

Invalid matching 2:

To check if both cases are valid, we can keep two hash maps for storing the current mapping 1. if ps[a] != b return false else put {a,b} to ps 2. if sp[b] != a return false else put {b,a} to sp

Time: $O(n)$ Space: $O(n)$

Code

class Solution {
public:
    bool wordPattern(string pattern, string s) {
        map<char, string> ps; // pattern to string
        map<string, char> sp; // string to pattern
        istringstream ss(s);
        int p = 0;
        for (string word; ss >> word; ++p) {
            char currp = pattern[p];
            if (ps.count(currp) && ps[currp] != word) return false;
            ps[currp] = word;
            if (sp.count(word) && sp[word] != currp) return false;
            sp[word] = currp;
        }
        return p == pattern.size();
    }
};

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Last update: April 1, 2022