Add 369. Plus One Linked List

linked_list/plus_one_linked_list.md

@@ -0,0 +1,62 @@
+# Plus One Linked List
+
+> Given a non-negative integer represented as non-empty a singly linked list of digits, plus one to the integer.
+
+> You may assume the integer do not contain any leading zero, except the number 0 itself.
+
+> The digits are stored such that the most significant digit is at the head of the list.
+
+> Given 1->2->3, 
+
+> return 1->2->4.
+
+此题计算方法与数组分类中的[Plus One](../array/plus_one.md)完全一样。需要注意两点：
+
+ + 单向链表没有办法像数组一样轻易从低位往高位操作，因此需要借助递归完成；
+ + 可使用指针传递进位。
+
+ ```c++
+ /**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* plusOne(ListNode* head) {
+        bool* carry = new bool(false);
+        recur(head, carry);
+
+        if(!(*carry)) {
+            return head;
+        }
+        else{
+            ListNode* newHead = new ListNode(1);
+            newHead->next = head;
+            return newHead;
+        }
+
+
+    }
+
+    ListNode* recur(ListNode* head, bool* carry) {
+        if(head->next != nullptr) {recur(head->next, carry);}
+
+        if(head->next == nullptr || *carry){
+            head->val++;
+            if(head->val >= 10) {
+                head->val = 0;
+                *carry=true;
+            }
+            else{
+                 *carry = false;
+            }
+        }       
+
+        return head;
+    }
+};
+```