23. Merge k Sorted Lists

Description

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

Solution

Divide&conquer

没啥好说的，使用归并思想两两merge。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        return mergeKLists(lists, 0, lists.length - 1);
    }
    
    public ListNode mergeKLists(ListNode[] lists, int start, int end) {
        if (start > end) {
            return null;
        }
        
        if (start == end) {
            return lists[start];
        }
        
        int mid = (start + end) >> 1;
        ListNode p = mergeKLists(lists, start, mid);
        ListNode q = mergeKLists(lists, mid + 1, end);
        return mergeSortedList(p, q);
    }
    
    public ListNode mergeSortedList(ListNode p, ListNode q) {
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        while (p != null && q != null) {
            if (p.val <= q.val) {
                tail.next = p;
                p = p.next;
            } else {
                tail.next = q;
                q = q.next;
            }
            tail = tail.next;
        }
        if (p != null) {
            tail.next = p;
        } else {
            tail.next = q;
        }
        return dummy.next;
    }
}

MinHeap

我不懂收益何在啊...TBD