reorder list

Author: zhongn2

ReorderList.cpp

@@ -0,0 +1,61 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode *findMid(ListNode *head){
+        if(head == NULL) return head;
+        ListNode *slow = head,*fast = head -> next;
+        while(fast && fast -> next){
+            slow = slow -> next;
+            fast = fast -> next -> next;
+        }
+        return slow;
+    }
+    
+  ListNode *reverseList(ListNode *head){
+        ListNode *prev = NULL;
+        while(head != NULL){
+            ListNode *tmp = head -> next;
+            head -> next = prev;
+            prev = head;
+            head = tmp;
+        }
+        return prev;
+    }
+      
+    
+    void mergeList(ListNode *head, ListNode *tail){
+        ListNode *dummy = new ListNode(-1);
+        int index = 0;
+        while(head && tail){
+            if(index %2 == 0){
+                dummy -> next = head;
+                head = head -> next;
+            }else{
+                dummy -> next = tail;
+                tail = tail -> next;
+            }
+            dummy = dummy -> next;// don't forget
+            index ++;
+        }
+        if(head != NULL) 
+            dummy -> next = head;
+        if(tail != NULL)
+            dummy -> next = tail;
+    }
+   
+    
+    void reorderList(ListNode *head) {
+        if(head == NULL) return;
+        ListNode *mid = findMid(head);
+        ListNode *tail = reverseList(mid ->next);
+        mid-> next = NULL; // break the head and tail
+        mergeList(head, tail);
+    }
+};