doc: add problem links

jtr109 · jtr109 · commit 03f7f6ff4aa1 · 2021-05-08T11:27:50.000+08:00
diff --git a/delete_operation_for_two_strings/src/lib.rs b/delete_operation_for_two_strings/src/lib.rs
@@ -1,3 +1,8 @@
+/*
+ * https://leetcode.com/explore/challenge/card/may-leetcoding-challenge-2021/598/week-1-may-1st-may-7th/3734/
+ * https://leetcode.com/problems/delete-operation-for-two-strings/
+*/
+
 #[cfg(test)]
 mod tests {
     #[test]
