Maximum Twin Sum of a Linked List

Description

In a linked list of size n, where n is even, the ith node (0-indexed) of the linked list is known as the twin of the (n-1-i)th node, if 0 <= i <= (n / 2) - 1.

• For example, if n = 4, then node 0 is the twin of node 3, and node 1 is the twin of node 2. These are the only nodes with twins for n = 4.

The twin sum is defined as the sum of a node and its twin.

Given the head of a linked list with even length, return the maximum twin sum of the linked list.

Example 1:

Input: head = [5,4,2,1] Output: 6 Explanation: Nodes 0 and 1 are the twins of nodes 3 and 2, respectively. All have twin sum = 6. There are no other nodes with twins in the linked list. Thus, the maximum twin sum of the linked list is 6.

Example 2:

Input: head = [4,2,2,3] Output: 7 Explanation: The nodes with twins present in this linked list are:

• Node 0 is the twin of node 3 having a twin sum of 4 + 3 = 7.
• Node 1 is the twin of node 2 having a twin sum of 2 + 2 = 4. Thus, the maximum twin sum of the linked list is max(7, 4) = 7.

Example 3:

Input: head = [1,100000] Output: 100001 Explanation: There is only one node with a twin in the linked list having twin sum of 1 + 100000 = 100001.

Constraints:

• The number of nodes in the list is an even integer in the range [2, 105].
• 1 <= Node.val <= 105

Code

Time Complexity: $O(N)$, Space Complexity: $O(1)$

和 Reorder List 類似，先切兩半，反轉後半段。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
int pairSum(struct ListNode* head) {
 
    struct ListNode* slow = head;
    struct ListNode* fast = head->next;
    while(fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
    }
 
    // isolate the frist half
    struct ListNode* temp = slow->next;
    slow->next = NULL;
 
    // reverse the second half
    struct ListNode* prev = NULL;
    while(temp) {
        struct ListNode* next = temp->next;
        temp->next = prev;
        prev = temp;
        temp = next;
    }
 
    int res = INT_MIN;
    while(prev && head) {
        int sum = prev->val + head->val;
        if(sum > res) {
            res = sum;
        }
        prev = prev->next;
        head = head->next;
    }
    return res;
 
}

Source

• Maximum Twin Sum of a Linked List - LeetCode