Palindrome List

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 ListNode* find(ListNode* A) {
     ListNode* slow = A;
     ListNode* fast = A;
     while(fast->next and fast->next->next) {
         fast = fast->next->next;
         slow = slow->next;
     }
     return slow;
 }
 ListNode* reverse(ListNode* A) {
     ListNode* dummy = new ListNode(-1);
     ListNode* runner = A;
     while(runner) {
         ListNode* next = runner->next;
         runner->next = dummy->next;
         dummy->next = runner;
         runner = next;
     }
     return dummy->next;
 }
int Solution::lPalin(ListNode* A) {
    if(A->next == NULL) return true;
    ListNode* mid = find(A);
    ListNode* cut = mid->next;
    mid->next = NULL;
    ListNode* rev = reverse(cut);
    ListNode* runner = A;
    while(rev) {
        if(rev->val != runner->val) return false;
        rev = rev->next;
        runner = runner->next;
    }
    return true;
}