LeetCode86 Partition List

题目：

Given a linked list and a value x, partition it such that all nodes less than x come before nodes greater than or equal to x.

You should preserve the original relative order of the nodes in each of the two partitions.（Medium）

For example,
Given 1->4->3->2->5->2 and x = 3,
return 1->2->2->4->3->5.

分析：

把链表归并，其思路就是先开两个链表，把小于x的值接在链表left后面，大于x的值接在链表right后面；

然后把链表left的尾部与链表right的头部接在一起。

注意：链表right的尾部next赋值为nullptr。

代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* partition(ListNode* head, int x) {
        ListNode dummy1(0);
        ListNode dummy2(0);
        ListNode* left = &dummy1;
        ListNode* right = &dummy2;
        while (head != nullptr) {
            if (head -> val < x) {
                left -> next = head;
                left = left -> next;
            }
            else {
                right -> next = head;
                right = right -> next;
            }
            head = head -> next;
        }
        left -> next = dummy2.next;
        right -> next = nullptr;
        return dummy1.next;
    }
};