feat(src): add 86-partition

Author: kwu130

src/86-partition.cpp

@@ -0,0 +1,55 @@
+//
+// Created by 1412 on 2025/11/18.
+//
+
+#include <bits/stdc++.h>
+#include "../include/common.h"
+
+using namespace std;
+using listHelper::ListNode;
+
+class Solution {
+public:
+    ListNode* partition(ListNode* head, int x) {
+        if (head == nullptr || head->next == nullptr) return head;
+        ListNode dummy1 = ListNode(-1);
+        ListNode dummy2 = ListNode(-1);
+        ListNode *p1 = &dummy1, *p2 = &dummy2;
+        while (head != nullptr) {
+            if (head->val < x) {
+                p1->next = head;
+                p1 = p1->next;
+            } else {
+                p2->next = head;
+                p2 = p2->next;
+            }
+            head = head->next;
+        }
+        p2->next = nullptr;
+        p1->next = dummy2.next;
+
+        return dummy1.next;
+    }
+};
+
+void test() {
+    Solution s;
+    ListNode* head = listHelper::build({1, 4, 3, 2, 5, 2});
+    ListNode* expected = listHelper::build({1, 2, 2, 4, 3, 5});
+    ListNode* res = s.partition(head, 3);
+    assert(listHelper::equal(res, expected));
+    listHelper::free(head);
+    listHelper::free(expected);
+
+    head = listHelper::build({2, 1});
+    expected = listHelper::build({1, 2});
+    res = s.partition(head, 2);
+    assert(listHelper::equal(res, expected));
+    listHelper::free(head);
+    listHelper::free(expected);
+}
+
+int main() {
+    test();
+    return 0;
+}