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class Solution {
    public int pairSum(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        slow = reverse(slow);
        fast = head;
        int ans = Integer.MIN_VALUE;
        while (slow != null) {
            int currSum = slow.val + fast.val;
            ans = Math.max(ans, currSum);
            slow = slow.next;
            fast = fast.next;
        }
        return ans;
    }

    private ListNode reverse(ListNode node) {
        ListNode prev = null, curr = node;
        while (curr != null) {
            ListNode next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
}

找中点, 反转, 遍历.

时间复杂度: O(n)
空间复杂度: O(1)