func isSubsequence(s string, t string) bool {
	if s == "" {
		return true
	}
	k, m, n := 0, len(s), len(t)
	for i := 0; i < n; i++ {
		if s[k] == t[i] {
			k++
			if k == m {
				return true
			}
		}
	}
	return false
}