Day-01- list Leetcode-206, 92, 160, 141, 86, 138, 21, 23

Example One: LeetCode206

//已知链表头节点指针head，将链表逆序，不可申请额外空间
#include <stdio.h>
struct ListNode {
       int val;
       ListNode* next;
       ListNode(int x) :val(x), next(NULL) {}
};
class Solution {
public:
       ListNode* reverseList(ListNode* head) {
              ListNode* new_head = NULL;
              while (head) {
                      ListNode* next = head->next;
                      head->next = new_head;
                      new_head = head;
                      head = next;
              }
              return new_head;
       }
};
int main() {
       ListNode a(1);
       ListNode b(2);
       ListNode c(3);
       ListNode d(4);
       ListNode e(5);
       a.next = &b;
       b.next = &c;
       c.next = &d;
       d.next = &e;
       Solution solve;
       ListNode* head = &a;
       printf("Before reverse:\n");
       while (head) {
              printf("%d\n", head->val);
              head = head->next;
       }
       head = solve.reverseList(&a);
       printf("After reverse:\n");
       while (head) {
              printf("%d\n", head -> val);
              head = head->next;
       }
       return 0;
}