add partition.cpp

Author: kamyu104

C++/partition.cpp

@@ -0,0 +1,35 @@
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+    public:
+        ListNode *partition(ListNode *head, int x) {
+            ListNode left_dummy(-1);
+            ListNode right_dummy(-1);
+            auto left_cur = &left_dummy;
+            auto right_cur = &right_dummy;
+
+            for(auto cur = head; cur; cur = cur->next) {
+                if(cur->val < x) {
+                    left_cur->next = cur;
+                    left_cur = cur;
+                }
+                else {
+                    right_cur->next = cur;
+                    right_cur = cur;
+                }
+            }
+
+            left_cur->next = right_dummy.next;
+            right_cur->next = nullptr;
+            return left_dummy.next;
+        }
+};