1. 題目

2. 解答

用一個單鏈表來實現，只有一個頭指標。因為不能建立哨兵結點，因此要特別注意是否在頭結點處操作。

class MyLinkedList {
public:
    
    struct ListNode {
        int val;
        ListNode *next;
        ListNode(int x) : val(x), next(NULL) {}
    };
    
    ListNode *head;
    
    /** Initialize your data structure here. */
    MyLinkedList() {
        head = NULL;
    }
    
    /** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
    int get(int index) {
        
        if (index < 0 || head == NULL) return -1;
        
        int i = 0;
        ListNode *temp = head;
        
        while(i < index && temp->next != NULL)
        {
            temp = temp->next;
            i++;
            if (i == index) break;
        }
        
        if (i == index) return temp->val;
        else    return -1;
    }
    
    /** 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. */
    void addAtHead(int val) {
        
        if (head == NULL)// 原連結串列沒有結點 
        {
            head = new ListNode(val);
        }
        else // 原連結串列有結點
        {
            ListNode *temp = new ListNode(val);
            temp->next = head;
            head = temp;
        } 
    }
    
    /** Append a node of value val to the last element of the linked list. */
    void addAtTail(int val) {
        
        ListNode *temp = head;
        
        if(temp == NULL) temp = new ListNode(val); // 原連結串列沒有結點
        else
        {
            // 原連結串列有結點，先找到到鏈尾再新增
            while(temp->next != NULL)
            {
                temp = temp->next;
            }
            temp->next = new ListNode(val);
        }  
    }
    
    /** 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 end of linked list. If index is greater than the length, the node will not be inserted. */
    void addAtIndex(int index, int val) {
        
        if (index < 0) return;
        if (head == NULL)
        {
            if (index > 0)
            { 
                return;
            }
            else
            {
                head = new ListNode(val);
                return;
            }
        }

        int i = 0;
        ListNode *temp = head;
        ListNode *last = NULL;

        // 查詢待插入結點及其上一個結點
        while(i < index && temp->next != NULL)
        {
            last = temp;
            temp = temp->next;
            i++;
            
            if (i == index) break;
        }
        
        if (i == index)    // 在待插入結點前插入
        {
            if (i == 0) // 在頭結點後插入
            {
                temp = new ListNode(val);
                temp->next = head;
                head = temp;
            }
            else // 在其他位置插入
            { 
                last->next = new ListNode(val);
                last->next->next = temp;
            }
        } 
        else if (i == index - 1)    // 在連結串列尾插入
        {
            temp->next = new ListNode(val);
        }
    }
    
    /** Delete the index-th node in the linked list, if the index is valid. */
    void deleteAtIndex(int index) {
        
        if (index < 0 || head == NULL) return;
        
        int i = 0;
        ListNode *temp = head;
        ListNode *last = NULL;

        // 查詢待刪除結點及其上一個結點
        while(i < index && temp->next != NULL)
        {
            last = temp;
            temp = temp->next;
            i++;
            
            if (i == index) break;
        }
        
        if (i == index)    
        {
            if (i == 0) // 刪除頭結點
            {
                delete head;
                head = NULL;
            }
            else  // 刪除其它結點
            {
                last->next = temp->next;
                delete temp;
            }
        } 
    }
};

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * MyLinkedList obj = new MyLinkedList();
 * int param_1 = obj.get(index);
 * obj.addAtHead(val);
 * obj.addAtTail(val);
 * obj.addAtIndex(index,val);
 * obj.deleteAtIndex(index);
 */
 

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