Implemented p19

src/p19_remove_nth_node_from_end_of_list.rs

@@ -1,9 +1,26 @@
 use super::linked_list::ListNode;
 
-struct Solution;
+pub struct Solution;
 impl Solution {
     pub fn remove_nth_from_end(head: Option<Box<ListNode>>, n: i32) -> Option<Box<ListNode>> {
-        return None;
+        let mut head = head.unwrap();
+        let mut node_before_nth: &mut Box<ListNode>;
+        // unsafe is required to have immutable and mutable references at the same time
+        // in order to avoid cloning the list altogether
+        unsafe { node_before_nth = &mut *(&mut head as *mut Box<ListNode>); };
+        let mut tail = &head;
+        for _ in 0..n-1 {
+            tail = tail.next.as_ref().unwrap();
+        }
+        if tail.next.is_none() { return head.next; }
+        else { tail = tail.next.as_ref().unwrap(); }
+        while tail.next.is_some() {
+            tail = tail.next.as_ref().unwrap();
+            node_before_nth = node_before_nth.next.as_mut().unwrap();
+        }
+        let node_to_remove = node_before_nth.next.as_mut().unwrap();
+        node_before_nth.next = node_to_remove.next.take();
+        return Some(head);
     }
 }
 
@@ -21,3 +38,8 @@ fn test2() {
 fn test3() {
     assert_eq!(Solution::remove_nth_from_end(list![1,2], 1), list![1]);
 }
+
+#[test]
+fn test4() {
+    assert_eq!(Solution::remove_nth_from_end(list![1,2], 2), list![2]);
+}