add problem and discuss link to existing solutions

Author: songyzh

src/solution/s0001_two_sum.rs

@@ -18,6 +18,9 @@
  */
 pub struct Solution {}
 
+// problem: https://leetcode.com/problems/two-sum/
+// discuss: https://leetcode.com/problems/two-sum/discuss/?currentPage=1&orderBy=most_votes&query=
+
 // submission codes start here
 
 use std::collections::HashMap;

src/solution/s0002_add_two_numbers.rs

@@ -19,6 +19,9 @@
 pub struct Solution {}
 use crate::util::linked_list::{to_list, ListNode};
 
+// problem: https://leetcode.com/problems/add-two-numbers/
+// discuss: https://leetcode.com/problems/add-two-numbers/discuss/?currentPage=1&orderBy=most_votes&query=
+
 // submission codes start here
 
 impl Solution {