Initialize your data structure here. */

@param head The linked list's head. Note that the head is guaranteed to be not null, so it contains at least one node. */

Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list. */

Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the en

Append a node of value val to the last element of the linked list. */

int addDigits(int num) { int sum=0; while(num>9) { while(num) { sum+=(num%10); num/=10; } num=sum; sum=0; } return num; }