2017-09-10 LeetCode_019 Remove Nth Node From End of List

19. Remove Nth Node From End of List

Given a linked list, remove the nth node from the end of list and return its head.

For example,

   Given linked list: 1->2->3->4->5, and n = 2.   After removing the second node from the end, the linked list becomes 1->2->3->5.

Note:
Given n will always be valid.
Try to do this in one pass.

Solution:

class Solution {

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public:

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    ListNode* removeNthFromEnd(ListNode* head, int n) {

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        if (head == NULL) return head;

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        ListNode *p = head;

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        int l = 0;

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        while (p != NULL) {

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            l++; p = p->next;

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        }

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        p = head;

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        if (n > l) return head;

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        if (n == l) {

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            ListNode *q = head;

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            head = q->next;

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            delete q;

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            return head;

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        } else {

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            int s = l-n;

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            ListNode *q = head->next;

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            while (--s) {

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                p = p->next; q = q->next;

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            }

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            p->next = q->next;

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            delete q;

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            return head;

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        }

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    }

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};