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;}