Leetcode 391, Is Subsequence

Given a string s and a string t, check if s is subsequence of t.

You may assume that there is only lower case English letters in both s and t. t is potentially a very long (length ~= 500,000) string, and s is a short string (<=100).

A subsequence of a string is a new string which is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (ie, "ace"is a subsequence of "abcde" while "aec" is not).

Example 1:
s = "abc", t = "ahbgdc"

Return true.

Example 2:
s = "axc", t = "ahbgdc"

Return false.

 

本题的思路比较简答，设置一个计数器，在scan list t的过程中，每发现一个s里面的字母，计数器就+1。如果计数器等于s的长度，说明已经找到一个subsequence 等于s。

class Solution(object):
    def isSubsequence(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: bool
        """
        if not s:
            return True
        i = 0
        for c in t:
            if c == s[i]:
                i += 1
            if i == len(s):
                return True
        
        return False

 

本题也可以用queue的思想，注意popleft() 和 collections.deque(s) 的使用。参考书影博客 http://bookshadow.com/weblog/2016/09/04/leetcode-is-subsequence/

 

 

class Solution(object):
    def isSubsequence(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: bool
        """
        queue = collections.deque(s)
        
        for x in t:
            if not queue:
                return True
            if queue[0] == x:
                queue.pop(0)
        
        return not queue