291. Word Pattern II

题目：

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:

1. pattern = "abab", str = "redblueredblue" should return true.
2. pattern = "aaaa", str = "asdasdasdasd" should return true.
3. pattern = "aabb", str = "xyzabcxzyabc" should return false.

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

链接：  http://leetcode.com/problems/word-pattern-ii/

题解：

题目跟Word Pattern基本一样，但输入str里面没有delimiter，所以我们要使用Backtracking来做。因为上一题使用了两个map，所以这一题延续使用，结果给自己造成了很大的困难，代码也写的长而难懂。二刷一定要好好研究backtracking，争取也写出简洁漂亮的代码。

Time Complexity - O(2ⁿ)， Space Complexity - O(2ⁿ)

public class Solution {
    public boolean wordPatternMatch(String pattern, String str) {
        if(pattern.length() == 0 && str.length() == 0) {
            return true;
        }
        if(pattern.length() == 0 || str.length() == 0) {
            return false;
        }
        Map<Character, String> patternToStr = new HashMap<>();
        Map<String, Character> strToPattern = new HashMap<>();
        return wordPatternMatch(pattern, str, patternToStr, strToPattern, 0, 0);
    }

    private boolean wordPatternMatch(String pattern, String str, Map<Character, String> patternToStr, Map<String, Character> strToPattern, int posPattern, int posString) {
        if(posPattern == pattern.length()) {
            return true;
        }
        if(str.length() == 0 && posPattern < pattern.length()) {
            return false;
        }

        int i = 0;
        for(i = posString; i < str.length(); i++) {
            String word = str.substring(posString, i + 1);
            if(posPattern >= pattern.length()) {
                return false;
            }
            char c = pattern.charAt(posPattern);    

            if(!patternToStr.containsKey(c) && !strToPattern.containsKey(word)) {
                patternToStr.put(c, word);
                strToPattern.put(word, c);
                if(wordPatternMatch(pattern, str.substring(i + 1), patternToStr, strToPattern, posPattern + 1, 0)) {
                    return true;
                }
                patternToStr.remove(c);
                strToPattern.remove(word);
            } else if(patternToStr.containsKey(c) && !word.equals(patternToStr.get(c))) {
                if(word.length() == patternToStr.get(c).length()) {
                    return false;
                }
            } else if(strToPattern.containsKey(word) && c != strToPattern.get(word)) {
            } else {
                posPattern++;
                posString += word.length();
            }
        }

        return posPattern == pattern.length() ? true: false;
    }
}

Reference:

https://leetcode.com/discuss/63252/share-my-java-backtracking-solution

https://leetcode.com/discuss/63393/20-lines-java-clean-solution-easy-to-understand

https://leetcode.com/discuss/63583/20-lines-concise-java-solution-with-explanation

https://leetcode.com/discuss/63724/super-easy-understand-backtracking-java-solution