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.

Examples:

pattern = abba, str = dog cat cat dog should return true. pattern = abba, str = dog cat cat fish should return false. pattern = aaaa, str = dog cat cat dog should return false. 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.

Think

• Firstly, you need to convert str into string array with spliting by space.
• Check two stuffs are equal length, if not? directly return false!
• Setup a hashmap for storing element both pattern and strs. However, the key are different types: character, string. Why we do not just use string? Consider about this case: pattern - abba, str - a a b a. Pattern and Word has the same kind of element. It cannot easily to distinguish.
• The value in hashmap should be the
• There is one more tricky thing: why we compare the map.put() return value? Here is a segment in HashMap API.

     if (e != null) { // existing mapping for key
        V oldValue = e.value;
        if (!onlyIfAbsent || oldValue == null)
            e.value = value;
            afterNodeAccess(e);
            return oldValue;
    }

• That means the previous index(value) of character(key) will be returned as index reference. So we return the previous index and also update the index for next index reference!
• So if the both returned index doesn't matched, it should return false.

Solution

public class Solution {
    public boolean wordPattern(String pattern, String str) {
        String[] strs = str.split(" ");
        if(strs.length != pattern.length())
            return false;
        Map<Object, Integer> map = new HashMap<>();
        for(int i = 0; i < strs.length; i++) {
            if(!Objects.equals(map.put(pattern.charAt(i), i), map.put(strs[i], i)))
                return false;
        }
        return true;
    }
}

Problem II

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 substring in str.

Examples

pattern = "abab", str = "redblueredblue" should return true.

pattern = "aaaa", str = "asdasdasdasd" should return true.

pattern = "aabb", str = "xyzabcxzyabc" should return false.

Notes

You may assume both pattern and str contains only lowercase letters.

Think

• Got all combinations and check any combination matched the pattern

Solution

public boolean wordPatternMatch(String pattern, String str) {
        // get all combinations
        List<String> combinations = new ArrayList<>();
        backtracking(combinations, str, "", 0);

        // check any matched case
        boolean res = false;
        for (String s : combinations) {
            res |= wordPattern(pattern, s);
        }
        return res;
    }

    private void backtracking(List<String> combinations, String str,
            String cur, int index) {
        if (index >= str.length()) {
            combinations.add(new String(cur));
            return;
        }

        for (int i = index; i < str.length(); i++) {
            String origin = cur;
            backtracking(combinations, str, cur + (cur.length() == 0 ? "" : " ")
                    + str.substring(index, i + 1), i + 1);
            cur = origin;
        }
    }