单词分割

牛客网：单词分割
题目描述
Given a string s and a dictionary of words dict, add spaces in s to construct a sentence where each word is a valid dictionary word.
Return all such possible sentences.

输入描述:
s =“catsanddog”
dict =“cat”, “cats”, “and”, “sand”, “dog”
输出描述:
[cats and dog, cat sand dog]

用递归来做：

1. 每次在dict中查找s的开头单词，如果找到符合条件的单词，就将其记录在结果中。
2. 找到合适的单词后，就从上次找到的单词的下一位开始递归查找。直至s为空字符为止。

import sys
def dfs(s, dic, arr, ans):
    if not s:
        ans.append(' '.join(arr))
    for w in dic:
        if s.startswith(w):
            dfs(s[len(w):], dic, arr + (w,), ans)
if __name__ == "__main__":
    s = raw_input().strip()
    s = s[s.index('"') + 1:-1]
    dic = raw_input().strip()
    dic = eval("{" + dic[dic.index('"'):] + "}")
    ans = []
    dfs(s, dic, tuple(), ans)
    print('[' + ', '.join(ans) + ']')