160相交链表

 思路将两个链表组合起来，这样长度就相等了，然后双指针遍历，O（m+n） O(1)

class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        if (headA == nullptr || headB == nullptr) {
            return nullptr;
        }
        ListNode *pA = headA, *pB = headB;
        while (pA != pB) {
            pA = pA == nullptr ? headB : pA->next;
            pB = pB == nullptr ? headA : pB->next;
        }
        return pA;
    }
};

哈希 

class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        unordered_setmyset;
        ListNode*left=headA;
        ListNode*right=headB;
        while(left!=nullptr){
            myset.insert(left);
            left=left->next;
        }
        while(right!=nullptr){
            if(myset.count(right))
                return right;
            right=right->next;
        }
        return nullptr;
    }
};