Leetcode 86. 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.
For example,
Given 1->4->3->2->5->2 and x = 3,
return 1->2->2->4->3->5.

分析

给定一个链表，要求将小于x的数字移到大于或等于x的节点之前。那么首先找到那个分界点，然后再讲之后小于x的点移到该分界点之后。直到所有的节点都已经处理。需要注意首节点的特殊情况处理。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* partition(struct ListNode* head, int x) {
    struct ListNode *p1=head,*p2=head,*temp=head;
    bool firstnode=true;
    while(p1!=NULL)
    {
        if(p1->val>=x)
            break;
        p2=p1;
        p1=p1->next;
    }
    if(p1!=head)
        firstnode=false;
    if(p1==NULL)
        return head;
    while(p1->next!=NULL)
    {
        if(p1->next->valnext->next;
                head=p1->next;
                head->next=p2;
                p1->next=temp;
                p2=head;
                firstnode=false;
            }
            else
            {
                temp=p1->next->next;
                p1->next->next=p2->next;
                p2->next=p1->next;
                p1->next=temp;
                p2=p2->next;
            }
            
        }
        else p1=p1->next;
    }
    return head;
}