剑指Offer12

25. 合并两个有序链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(l1 == nullptr) return l2;
        if(l2 == nullptr) return l1;
        ListNode* mlist = new ListNode(0);
        ListNode* list = mlist; 
        while(l1 && l2)
        {
            if(l1->val < l2->val)
            {
                list->next = l1;
                l1 = l1->next;
            }else{
                list->next = l2;
                l2 = l2->next;
            }
            list = list->next;
        }
        if(l1)
            list->next = l1;
        else
            list->next = l2;
        return mlist->next;
    }
};

52. 两个链表的第一个公共节点

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        if(headA == nullptr || headB == nullptr) return nullptr;
        int l1 = getLength(headA);
        int l2 = getLength(headB);
        int l = 0;
        ListNode* fast = nullptr, *slow=nullptr;
        if(l1 > l2){
            l = l1 -l2;
            fast = headA;
            slow = headB;
        }else{
            l = l2 - l1;
            fast = headB;
            slow = headA;
        }
        while(l--){
            fast = fast->next;
        }
        while(fast!=nullptr && slow != nullptr && fast != slow){
            fast = fast->next;
            slow = slow->next;
        }
        ListNode* node = fast;
        return node;
    }

    int getLength(ListNode* head)
    {
        int length = 0;
        ListNode* cur = head;
        while(cur)
        {
            ++length;
            cur = cur->next;
        }
        return length;
    }
};