[C++] priority_queue custom compare method

If you can not edit the ListNode struct, you also can custom your own compare method of priority_queue. Just do something like this:

struct cmp {
    bool operator()(const ListNode* a, const ListNode* b) const {
        return a->val > b->val;
    }
};

When create a priority queue, you can use the struct of 'cmp' for the priority queue.

priority_queue<ListNode*, vector<ListNode*>, cmp> q;