86. Partition List

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本文介绍了一种链表分区算法，该算法将链表中小于给定值x的节点置于大于等于x的节点之前，同时保持原有相对顺序不变。示例中给出1->4->3->2->5->2，在x=3的情况下返回1->2->2->4->3->5。通过使用二级指针实现这一目标。

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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.

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

直接使用二级指针即可，解法如下：

class Solution {
public:
    ListNode* partition(ListNode* head, int x) {
        ListNode* less = NULL;
        ListNode** less_pp = &less;
        ListNode* not_less = NULL;
        ListNode** not_less_pp = &not_less;

        while(head != NULL){
            if(head->val < x){
                *less_pp = head;
                less_pp = &(head->next);
            }
            else{
                *not_less_pp = head;
                not_less_pp = &(head->next);
            }
            head = head->next;
        }

        if(less != NULL){
            *less_pp = not_less;
            *not_less_pp = NULL; //注意我们需要把not_less分支最后一各节点赋空，负责会死循环
        }
        else
            less = not_less;

        return less;
    }
};