Word Pattern


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.


  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.


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


本题跟 "Isomorphic Strings" 很类似,用两个HashMap, 记录从字符到字符串和字符串到字符的映射。


// Word Pattern
// Time Complexity: O(n), Space Complexity: O(n)
public class Solution {
    public boolean wordPattern(String pattern, String str) {
        String[] words = str.split(" ");
        if (words.length != pattern.length()) return false;

        final Map<Character, String> map1 = new HashMap<>();
        final Map<String, Character> map2 = new HashMap<>();
        for (int i = 0; i < words.length; ++i) {
            final Character key1 = pattern.charAt(i);
            if (map1.containsKey(key1)) {
                final String value = map1.get(key1);
                if (!value.equals(words[i])) return false;
            } else {
                map1.put(key1, words[i]);

            final String key2 = words[i];
            if (map2.containsKey(key2)) {
                final char value = map2.get(key2);
                if (value != pattern.charAt(i)) return false;
            } else {
                map2.put(key2, pattern.charAt(i));
        return true;


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