docs: Add explanation of time and space complexity

Author: taekwon-dev

valid-anagram/taekwon-dev.java

@@ -1,3 +1,10 @@
+/**
+ * 시간 복잡도: O(NlogN)
+ *   - Arrays.sort() > Dual-Pivot QuickSort
+ *
+ * 공간 복잡도: O(N)
+ *
+ */
 class Solution {
     public boolean isAnagram(String s, String t) {
         char[] sChars = s.toCharArray();