剑指Offer——反转链表

Question

Solution

Code

/*
struct ListNode {
int val;
struct ListNode *next;
ListNode(int x) :
val(x), next(NULL) {
}
};*/
class Solution {
public:
// 就地完成反转
ListNode* ReverseList(ListNode* pHead) {
ListNode* pre = NULL;
ListNode* head = pHead;
ListNode* next = head;

while (next) {
next = head->next;
head->next = pre;
pre = head;
if (next)
head = next;
}
return head;
}

// O(n)的空间
ListNode* ReverseList(ListNode* pHead) {
if (pHead == NULL)
return NULL;

stack<ListNode*> stack1;
ListNode* tmp = pHead;
while (tmp) {
stack1.push(tmp);
tmp = tmp->next;
}

ListNode* first = new ListNode(-1);
pHead = first;
while (!stack1.empty()) {
ListNode* current = stack1.top();
stack1.pop();
first->next = current;
first = current;

}
first->next = NULL;

return pHead->next;
}
};