lintcode-96-链表划分

96-链表划分

给定一个单链表和数值x，划分链表使得所有小于x的节点排在大于等于x的节点之前。
你应该保留两部分内链表节点原有的相对顺序。

样例

给定链表 1->4->3->2->5->2->null，并且 x=3
返回 1->2->2->4->3->5->null

标签

链表 两根指针

思路

遍历一次链表，使用常数级的额外空间，具体做法是：
使用两个指针lessNext，moreNext重新规划链表。使比 x 小的节点和比 x 大的节点重新组成两个新的链表，lessHead，moreHead是这两个新链表的头结点指针，然后，合并这两个链表即可。

code

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @param x: an integer
     * @return: a ListNode 
     */
    ListNode *partition(ListNode *head, int x) {
        // write your code here
        if(head == NULL) {
            return head;
        }
        ListNode *lessHead = NULL, *lessNext = NULL;
        ListNode *moreHead = NULL, *moreNext = NULL;
        ListNode *current = head;
        bool isLessHead = true, isMoreHead = true;

        while(current != NULL) {
            if(current->val < x) {
                if(isLessHead){
                    lessHead = lessNext = current;
                    isLessHead = false;
                }
                else {
                    lessNext->next = current;
                    lessNext = lessNext->next;
                }
            }
            else {
                if(isMoreHead){
                    moreHead = moreNext = current;
                    isMoreHead = false;
                }
                else {
                    moreNext->next = current;
                    moreNext = moreNext->next;
                }
            }
            current = current->next;
        }

        if(lessNext != NULL && moreNext != NULL) {
            lessNext->next = moreHead;
            moreNext->next = NULL;
            return lessHead;
        }
        else if(lessNext == NULL){
            moreNext->next = NULL;
            return moreHead;
        }
        else if(moreNext == NULL) {
            lessNext->next = NULL;
            return lessHead;
        }
    }
};