Merge k Sorted Lists[leetcode] 2种方法:堆实现&一般方法

本文介绍了一种使用最小堆优化合并多个链表的方法，通过实例代码演示了如何在链表合并过程中避免超时错误，提高了算法效率。

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本题用普通方法比较简单

注意如果一个ListNode*为NULL，应及时删除，否则会TLE

 ListNode *mergeKLists(vector<ListNode *> &lists) {
		ListNode * dummy = new ListNode(0);
		ListNode * cur = dummy;
		while (true)
		{
			int min = INT_MAX;
			int index = -1;
			for (int i = lists.size() - 1; i >= 0; i--)
			{
			    if (lists[i] != NULL && lists[i]->val < min)
				{
					min = lists[i]->val;
					index = i;
				}
			}
			if (index == -1) break;
			
		    cur->next = new ListNode(min);
		    cur = cur->next;
			lists[index] = lists[index]->next;
			if (lists[index] == NULL) 
			    lists.erase(lists.begin() + index);
		}
		ListNode * head = dummy->next;
		delete dummy;
		return head;
	}

第二种方法用到最小堆，使用make_heap, push_heap, pop_heap

class Solution {
    struct MyGreater
    {
        bool operator() (pair<ListNode*, int>& l, pair<ListNode*, int>& r)
        {
            return l.first->val > r.first->val;
        }
    };
public:
    ListNode *mergeKLists(vector<ListNode *> &lists) {
        vector<pair<ListNode*, int>> heap;
        int heapSize = lists.size();
        for (int i = heapSize - 1; i >= 0; i--)
        {
            if (lists[i] == NULL)
            {
                heapSize--;
                continue;
            }
            heap.push_back(make_pair(lists[i], i));
            lists[i] = lists[i]->next;
        }
        if (heapSize <= 0) return NULL;
        make_heap(heap.begin(), heap.begin() + heapSize, MyGreater());
        ListNode * root = new ListNode(0);
        ListNode * cur = root;
       
        while (true)
        {
            pop_heap(heap.begin(), heap.begin() + heapSize, MyGreater());
            cur->val = heap[heapSize - 1].first->val;
            int heapNum = heap[heapSize - 1].second;
            if (lists[heapNum] != NULL)
            {
                heap[heapSize - 1].first->val = lists[heapNum]->val;
                lists[heapNum] = lists[heapNum]->next;
                push_heap(heap.begin(), heap.begin() + heapSize, MyGreater());
            }
            else
            {
                heapSize--;
                if (heapSize == 0)
                {
                    return root;
                }
            }
            cur->next = new ListNode(0);
            cur = cur->next;
        }
        return NULL;
    }
};

pop_heap其实并不是将堆顶删除，而是将堆顶放在了vector中size()-1的位置，并排序0-size()-2。

push_heap是假设0-size()-2已经是一个堆了，将vector末尾元素插入到堆中。

我写的时候使用heapSize记录当前堆的大小，并且使用pair<ListNode*, int>记录某个元素对应在lists中的位置