Merge k Sorted Lists
<p>You are given an array of <code>k</code> linked-lists <code>lists</code>, each linked-list is sorted in ascending order.</p>

<p><em>Merge all the linked-lists into one sorted linked-list and return it.</em></p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> lists = [[1,4,5],[1,3,4],[2,6]]
<strong>Output:</strong> [1,1,2,3,4,4,5,6]
<strong>Explanation:</strong> The linked-lists are:
[
  1-&gt;4-&gt;5,
  1-&gt;3-&gt;4,
  2-&gt;6
]
merging them into one sorted linked list:
1-&gt;1-&gt;2-&gt;3-&gt;4-&gt;4-&gt;5-&gt;6
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> lists = []
<strong>Output:</strong> []
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> lists = [[]]
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>k == lists.length</code></li>
	<li><code>0 &lt;= k &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= lists[i].length &lt;= 500</code></li>
	<li><code>-10<sup>4</sup> &lt;= lists[i][j] &lt;= 10<sup>4</sup></code></li>
	<li><code>lists[i]</code> is sorted in <strong>ascending order</strong>.</li>
	<li>The sum of <code>lists[i].length</code> will not exceed <code>10<sup>4</sup></code>.</li>
</ul>
Linked List; Divide and Conquer; Heap (Priority Queue); Merge Sort
// Definition for singly-linked list.
// #[derive(PartialEq, Eq, Clone, Debug)]
// pub struct ListNode {
//   pub val: i32,
//   pub next: Option<Box<ListNode>>
// }
//
// impl ListNode {
//   #[inline]
//   fn new(val: i32) -> Self {
//     ListNode {
//       next: None,
//       val
//     }
//   }
// }
use std::cmp::Ordering;
use std::collections::BinaryHeap;

impl PartialOrd for ListNode {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}
impl Ord for ListNode {
    fn cmp(&self, other: &Self) -> Ordering {
        self.val.cmp(&other.val).reverse()
    }
}
impl Solution {
    pub fn merge_k_lists(lists: Vec<Option<Box<ListNode>>>) -> Option<Box<ListNode>> {
        let mut pq = lists
            .into_iter()
            .filter_map(|head| head)
            .collect::<BinaryHeap<_>>();
        let mut head = None;
        let mut cur = &mut head;
        while let Some(node) = pq.pop() {
            cur = &mut cur.insert(Box::new(ListNode::new(node.val))).next;
            if let Some(next) = node.next {
                pq.push(next);
            }
        }
        head
    }
}
