is_subsequence.go

/*
392. Is Subsequence
https://leetcode.com/problems/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.

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.
*/

package issubsequence

// greedy
// Time complexity: O(n), where n is min(  len(s),   len(t)    )
// Space complexity: O(1)
func isSubsequence(s string, t string) bool {
	var si, ti int

	for si < len(s) && ti < len(t) {
		if s[si] == t[ti] {
			si++
			ti++
		} else {
			ti++
		}
	}
	return si == len(s)
}