Solve a List problem in LeetCode Daily Challenge.

Author: zh-plus

LeetCode/daily_challenge/CMakeLists.txt

@@ -1 +1 @@
-add_executable(daily_challenge p_2021_3_26_PacificAtlanticWaterFlow.cpp)
+add_executable(daily_challenge p2021_4_1_PalindromeLinkedList.cpp)

LeetCode/daily_challenge/p_2021_3_26_PacificAtlanticWaterFlow.cpp renamed to LeetCode/daily_challenge/p2021_3_26_PacificAtlanticWaterFlow.cpp

@@ -0,0 +1,91 @@
+//
+// Created by 10578 on 2021/4/1.
+//
+
+#include "iostream"
+#include "vector"
+#include "algorithm"
+#include "initializer_list"
+
+using namespace std;
+
+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) {}
+
+    ListNode(std::initializer_list<int> ini) {
+        ListNode *prev, *curr = this;
+        for (auto &&x: ini) {
+            curr->val = x;
+            curr->next = new ListNode();
+            prev = curr;
+            curr = curr->next;
+        }
+        prev->next = nullptr;
+    }
+
+    void print() {
+        ListNode *curr = this;
+        while (curr) {
+            std::cout << curr->val << " ";
+            curr = curr->next;
+        }
+    }
+};
+
+class Solution {
+public:
+    bool isPalindrome(ListNode *head) {
+        if (head->next == nullptr) {
+            return true;
+        }
+
+        //O(1) space complexity
+        ListNode *mid = head, *tail = head;
+        while (tail) {
+            tail = tail->next;
+            if (tail && tail->next) {
+                mid = mid->next;
+                tail = tail->next;
+            }
+        }
+
+        ListNode *reversed_tail = mid->next;
+        ListNode *curr = reversed_tail->next;
+        reversed_tail->next = nullptr;
+        ListNode *prev = reversed_tail;
+        ListNode *next;
+        while (curr) {
+            next = curr->next;
+            curr->next = prev;
+            prev = curr;
+            curr = next;
+        }
+
+        ListNode *reversed_head = prev;
+        ListNode *rev_curr = reversed_head;
+        ListNode *for_curr = head;
+        while (rev_curr) {
+            if (rev_curr->val != for_curr->val) {
+                return false;
+            }
+            rev_curr = rev_curr->next;
+            for_curr = for_curr->next;
+        }
+
+        return true;
+    }
+};
+
+int main() {
+    ListNode *head = new ListNode{1};
+
+    Solution s;
+    cout << s.isPalindrome(head);
+}