86. Partition List

這題不難, 就是create兩條list然後, 大於等於目標值得就加到AList, 反之就加BList,

這題要注意的是, 如果你是直接把節點當作只到的對象, 在最後串起來的同時, 一定要讓AList的最後一個next指到 nullptr, 否則很有可能最後一個next其實是BList的值, 造成runtime error

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* partition(ListNode* head, int x) {
        ListNode *t1 = new ListNode(-1);
        ListNode *t2 = new ListNode(-1);
        ListNode *t2_h = t2, *t1_h = t1;
        while(head){
            ListNode *t = head->next;
            
            if(head->val >= x){
                t2->next = head;
                t2 = t2->next;
            }else{
                t1->next = head;
                t1 = t1->next;
            }
            head = t;
           
        }
        t2->next = nullptr;
        t1->next = t2_h->next;
       return t1_h->next;
    }
  
};