[Leetcode] Remove Duplicates from Sorted List

题目：

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3.

思路：两个指针，从前向后检查。

class Solution {
public:
    ListNode *deleteDuplicates(ListNode *head) {
        if (head == nullptr) return head;
        ListNode* back = head;
        ListNode* front = back->next;
        while (back != nullptr) {
            while (front != nullptr && front->val == back->val) {
                front = front->next;
            }
            back->next = front;
            back = front;
        }
        return head;
    }
};

总结：复杂度为O(n).