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描述
Given a string s and an array of strings words, determine whether s is a prefix string of words.
A string s is a prefix string of words if s can be made by concatenating the first k strings in words for some positive k no larger than words.length.
Return true if s is a prefix string of words, or false otherwise.
Example 1:
Input: s = "iloveleetcode", words = ["i","love","leetcode","apples"]
Output: true
Explanation:
s can be made by concatenating "i", "love", and "leetcode" together.
Example 2:
Input: s = "iloveleetcode", words = ["apples","i","love","leetcode"]
Output: false
Explanation:
It is impossible to make s using a prefix of arr.
Note:
1 <= words.length <= 100
1 <= words[i].length <= 20
1 <= s.length <= 1000
words[i] and s consist of only lowercase English letters.
解析
根据题意，给出了一个字符串 s ，还有一个单词列表 words ，让我们判断 s 是否是 words 的前缀字符串。题目中给出了定义判断字符串 s 是否是列表 words 的前缀，只需要判断 s 是否和 words 中的前 k 个字符串拼接起来的字符串相同即可，k 要小于 words 的长度。
其实思路已经出来了：
- 初始化一个需要不断拼接的字符串 prefix
- 遍历 words 中的单词，将当前的单词 word 拼接到 prefix 后面，判断此时的 prefix 如果和 s 相等，直接返回 True ，否则重复此过程
- 遍历结束如果没有返回 True ，则表示没有找到 prefix ，直接返回 False
解答
class Solution(object):
def isPrefixString(self, s, words):
"""
:type s: str
:type words: List[str]
:rtype: bool
"""
prefix = ''
for word in words:
prefix += word
if s==prefix:
return True
return False
运行结果
Runtime: 20 ms, faster than 93.69% of Python online submissions for Check If String Is a Prefix of Array.
Memory Usage: 13.4 MB, less than 71.17% of Python online submissions for Check If String Is a Prefix of Array.
解析
另外，我们可以直接将 words 中的单词拼接成一个字符串 r ，然后判断 s 是否是 r 的前缀。但是要保证不能是“伪前缀”，如 s=a ，words=["aa","aaaa","banana"] ，这种输入应该是 False ，所以还要保证 s 的长度必须是合理的。原理一样，换汤不换药。
解答
class Solution(object):
def isPrefixString(self, s, words):
"""
:type s: str
:type words: List[str]
:rtype: bool
"""
lengths = [len(words[0])]
for word in words[1:]:
lengths.append(len(word)+lengths[-1])
return len(s) in lengths and ''.join(words).startswith(s)
运行结果
Runtime: 20 ms, faster than 93.69% of Python online submissions for Check If String Is a Prefix of Array.
Memory Usage: 13.6 MB, less than 22.97% of Python online submissions for Check If String Is a Prefix of Array.
原题链接：leetcode.com/problems/ch…
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