290. Word Pattern

Problem:

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Given a pattern and a string str, find if str 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 str.

Examples:

1. pattern = "abba", str = "dog cat cat dog" should return true.
2. pattern = "abba", str = "dog cat cat fish" should return false.
3. pattern = "aaaa", str = "dog cat cat dog" should return false.
4. pattern = "abba", str = "dog dog dog dog" should return false.

Notes:
You may assume pattern contains only lowercase letters, and str contains lowercase letters separated by a single space.

Analysis:

The difficulty of this problem lies in that how to decide new word in new pattern character not exist before. "abba" "dog cat cat dog". If there is a new pattern char, we need to make sure its paired string not exist before. Here we can use HashMap.containsValue().

Solution:

class Solution {
    public boolean wordPattern(String pattern, String str) {
        Map<Character, String> map = new HashMap<>();
        String[] strs = str.split(" ", -1);
        if (pattern.length() != strs.length) 
            return false;
        for (int i = 0; i < strs.length; i++) {
            char c = pattern.charAt(i);
            String word = strs[i];
            if (map.containsKey(c) && !map.get(c).equals(word))
                return false;
            if (!map.containsKey(c) && map.containsValue(word)) 
                return false;
            map.put(c, word);
        }
        return true;
    }
}