[leetcode] 92. Reverse Linked List II

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:

Given 

1->2->3->4->5->NULL

, m = 2 and n = 4,

return 

1->4->3->2->5->NULL

.

Note:

Given m, n satisfy the following condition:

1 ≤ m ≤ n ≤ length of list.

解法一：

感觉不应该是medium。

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseBetween(ListNode* head, int m, int n) {
if(!head) return NULL;
int idx = 1;

ListNode* dummy = new ListNode(-1);
dummy->next = head;

ListNode* pre = dummy, *cur = head;
while(idx<m) {
pre = cur;
cur = cur->next;
idx++;
}

while(idx<n){
ListNode* next = cur->next;
cur->next = next->next;
next->next = pre->next;
pre->next = next;
idx++;
}
return dummy->next;

}
};