139. Word Break.py

"""
Given a non-empty string s and a dictionary wordDict containing a list of non-empty words, determine if s can be segmented into a space-separated sequence of one or more dictionary words.

Note:

The same word in the dictionary may be reused multiple times in the segmentation.
You may assume the dictionary does not contain duplicate words.
Example 1:

Input: s = "leetcode", wordDict = ["leet", "code"]
Output: true
Explanation: Return true because "leetcode" can be segmented as "leet code".
Example 2:

Input: s = "applepenapple", wordDict = ["apple", "pen"]
Output: true
Explanation: Return true because "applepenapple" can be segmented as "apple pen apple".
             Note that you are allowed to reuse a dictionary word.
Example 3:

Input: s = "catsandog", wordDict = ["cats", "dog", "sand", "and", "cat"]
Output: false
"""
class Solution:
    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
        visited = set()
        q = collections.deque([0])
        while q:
            start = q.popleft()
            if start not in visited:
                for end in range(start+1,len(s)+1):
                    if s[start:end] in wordDict:
                        if end == len(s):
                            return True
                        else:
                            q.append(end)
                visited.add(start)
        return False