Q2.4 Partition List

Q:Write code to partiton a linked list around a value x, such that all nodes less than x come before all nodes greater than or equal to x.

A:双链表,其中一个记录小于x的置的节点，剩下的节点记录在第一个链表下，最后将第一个链表的尾节点指向第二个链表的head

#include <iostream>
using namespace std;

struct ListNode {
    int val;
	ListNode *next;
 	ListNode(int x) : val(x), next(NULL) {}
 };
ListNode *init(int a[], int n) {
 	ListNode *head = NULL;
 	ListNode *p = NULL;
 	for (int i = 0; i < n; i++) {
 		ListNode *cur = new ListNode(a[i]);
 		if (i == 0) {
 			head = cur;
 			p = cur;
 		}
 		p->next = cur;
 		p = cur;
 	}
 	return head;
}
ListNode *partition(ListNode *head, int x) {
        ListNode small_dummy(-1);
        ListNode big_dummy(-1);
        ListNode *small = &small_dummy;
        ListNode *big = &big_dummy;
        for (ListNode *cur = head; cur; cur = cur->next) {
            if (cur->val < x) {
                small->next = cur;
                small = small->next;
            } else {
                big->next = cur;
                big = big->next;
            }
        }
        small->next = big_dummy.next;
        big->next = NULL;
        
        return small_dummy.next;
}
void printList(ListNode *head) {
	ListNode *p = head;
	for( ; p; p = p->next) {
		cout<<p->val<<" ";
	}
	cout<<endl;
}
int main() {
	int a[10] = {1,4,3,2,5,2};
	ListNode *head = init(a, 6);
	printList(head);
	head = partition(head, 3);
	printList(head);
	return 0;
}