add: 220308 [cpp]

cpp/2203/220308.cpp

@@ -0,0 +1,54 @@
+#include <iostream>
+#include <cassert>
+
+/**
+ * Definition for singly-linked list.
+ */
+struct ListNode {
+    int val;
+    ListNode* next;
+    explicit ListNode(int x, ListNode* n = nullptr) : val(x), next(n) {}
+    ~ListNode() { delete this->next; this->next = nullptr; }
+};
+
+class Solution {
+private:
+    /**
+     * Solution for problem # 142
+     * Refer: 220119.cpp
+     * @param head Head of linked "list"
+     * @return Pointer to where the cycle begins
+     */
+    static ListNode* detectCycle(const ListNode* head) {
+        if (!(head && head->next))
+            return nullptr;
+        const auto* fast = head->next->next;
+        const auto* slow = head->next;
+        for (; fast && fast->next && fast != slow; ) {
+            fast = fast->next->next;
+            slow = slow->next;
+        }
+        if (!(fast && fast->next))
+            return nullptr;
+        assert(fast == slow);
+        int cycleLength = 1;
+        for (fast = fast->next; fast != slow; ++cycleLength, (fast = fast->next));
+        fast = slow = head;
+        for (int i = 0; i < cycleLength; ++i)
+            fast = fast->next;
+        if (fast == slow) {
+            for (int i = 1; i < cycleLength; ++i)
+                fast = fast->next;
+        } else {
+            while (fast->next != slow->next) {
+                fast = fast->next;
+                slow = slow->next;
+            }
+        }
+        return const_cast<ListNode*>(fast->next);
+    }
+public:
+    static bool hasCycle(ListNode *head) {
+        return detectCycle(head) != nullptr;
+    }
+};

cpp/2203/CMakeLists.txt

@@ -3,4 +3,4 @@ PROJECT(2203)
 
 SET(CMAKE_CXX_STANDARD 23)
 
-ADD_EXECUTABLE(2203 220307-CN.cpp)
+ADD_EXECUTABLE(2203 220308.cpp)