Create 25.cpp

LeetCode-CN/25.cpp

@@ -0,0 +1,70 @@
+#include <stack>
+class Solution {
+public:
+    ListNode * reverseKGroup(ListNode* head, int k) {
+        ListNode* seg_start = head;
+        ListNode* now = head;
+        ListNode* return_head = head;
+        ListNode* last_seg = NULL;
+        int passed = 0;
+
+        while (now != NULL)
+        {
+            passed++;
+            if (passed == k)
+            {
+                // Meet a segment
+                ListNode* seg_end = now;
+                // Change head
+                if (seg_start == head)
+                {
+                    return_head = seg_end;
+                }
+
+                ListNode* p = seg_start;
+                std::stack<ListNode*> stk;
+                while (p != seg_end)
+                {
+                    stk.push(p);
+                    p = p->next;
+                }
+                // p is now seg_end
+
+                // checkpoint
+                ListNode* next_seg_start = seg_end->next;
+
+                while (!stk.empty())
+                {
+                    p->next = stk.top();
+                    stk.pop();
+                    p = p->next;
+                }
+
+                // stack is empty. p is now seg_start. 
+                // p->next now pointes to the orignal "next"
+                
+                // set last segment
+                if (last_seg)
+                {
+                    last_seg->next = seg_end;
+                }
+                last_seg = seg_start;
+                // set it to next_seg_start
+                p->next = next_seg_start;
+
+                seg_start = next_seg_start;
+                now = next_seg_start;
+                passed = 0;
+
+                // debug
+                //printList(return_head);
+            }
+            else
+            {
+                now = now->next;
+            }
+        }
+
+        return return_head;
+    }
+};