problems: add partition list

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/partition-list.cpp

@@ -0,0 +1,56 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+    ListNode* update_tail(ListNode* tail, ListNode* node) const
+    {
+        if (tail != nullptr) {
+            tail->next = node;
+        }
+        return node;
+    }
+
+public:
+    ListNode* partition(ListNode* head, int x)
+    {
+        ListNode* left_head = nullptr;
+        ListNode* right_head = nullptr;
+
+        ListNode* left_tail = nullptr;
+        ListNode* right_tail = nullptr;
+
+        while (head != nullptr) {
+            auto next_head = head->next;
+
+            if (head->val < x) {
+                left_tail = update_tail(left_tail, head);
+            } else {
+                right_tail = update_tail(right_tail, head);
+            }
+
+            if (left_head == nullptr) {
+                left_head = left_tail;
+            }
+            if (right_head == nullptr) {
+                right_head = right_tail;
+            }
+
+            head = next_head;
+        }
+
+        left_tail = update_tail(left_tail, right_head);
+        right_tail = update_tail(right_tail, nullptr);
+
+        if (left_head == nullptr) {
+            return left_tail;
+        }
+        return left_head;
+    }
+};