392. Is Subsequence

题目

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:

Input: s = "abc", t = "ahbgdc"
Output: true

Example 2:

Input: s = "axc", t = "ahbgdc"
Output: false

Follow up: If there are lots of incoming S, say S1, S2, ... , Sk where k >= 1B, and you want to check one by one to see if T has its subsequence. In this scenario, how would you change your code?

Credits: Special thanks to @pbrother for adding this problem and creating all test cases.

题目大意

给定字符串 s 和 t ，判断 s 是否为 t 的子序列。你可以认为 s 和 t 中仅包含英文小写字母。字符串 t 可能会很长（长度 ~= 500,000），而 s 是个短字符串（长度 <=100）。字符串的一个子序列是原始字符串删除一些（也可以不删除）字符而不改变剩余字符相对位置形成的新字符串。（例如，"ace"是"abcde"的一个子序列，而"aec"不是）。

解题思路

• 给定 2 个字符串 s 和 t，问 s 是不是 t 的子序列。注意 s 在 t 中还需要保持 s 的字母的顺序。
• 这是一题贪心算法。直接做即可。

代码

package leetcode

// 解法一 O(n^2)
func isSubsequence(s string, t string) bool {
	index := 0
	for i := 0; i < len(s); i++ {
		flag := false
		for ; index < len(t); index++ {
			if s[i] == t[index] {
				flag = true
				break
			}
		}
		if flag == true {
			index++
			continue
		} else {
			return false
		}
	}
	return true
}

// 解法二 O(n)
func isSubsequence1(s string, t string) bool {
	for len(s) > 0 && len(t) > 0 {
		if s[0] == t[0] {
			s = s[1:]
		}
		t = t[1:]
	}
	return len(s) == 0
}

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