leetcode 1961. Check If String Is a Prefix of Array（python）

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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.
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Example 2:

Input: s = "iloveleetcode", words = ["apples","i","love","leetcode"]
Output: false
Explanation:
It is impossible to make s using a prefix of arr.
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Note:

1 <= words.length <= 100
1 <= words[i].length <= 20
1 <= s.length <= 1000
words[i] and s consist of only lowercase English letters.
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解析

根据题意，给出了一个字符串 s ，还有一个单词列表 words ，让咱们判断 s 是不是 words 的前缀字符串。题目中给出了定义判断字符串 s 是不是列表 words 的前缀，只须要判断 s 是否和 words 中的前 k 个字符串拼接起来的字符串相同便可，k 要小于 words 的长度。

其实思路已经出来了：

• 初始化一个须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 若是和 s 相等，直接返回 True ，不然重复此过程
• 遍历结束若是没有返回 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
        	      
		
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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.
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解析

另外，咱们能够直接将 words 中的单词拼接成一个字符串 r ，而后判断 s 是不是 r 的前缀。可是要保证不能是“伪前缀”，如 s=a ，words=["aa","aaaa","banana"] ，这种输入应该是 False ，因此还要保证 s 的长度必须是合理的。原理同样，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)           	      
		
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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.
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原题连接：leetcode.com/problems/ch…

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