Leetcode-1019. 链表中的下一个更大节点 -Python

题目

给出一个以头节点 head 作为第一个节点的链表。链表中的节点分别编号为：node_1, node_2, node_3, … 。
每个节点都可能有下一个更大值（next larger value）：对于 node_i，如果其 next_larger(node_i) 是 node_j.val，那么就有 j > i 且 node_j.val > node_i.val，而 j 是可能的选项中最小的那个。如果不存在这样的 j，那么下一个更大值为 0 。
返回整数答案数组 answer，其中 answer[i] = next_larger(node_{i+1}) 。
注意：在下面的示例中，诸如 [2,1,5] 这样的输入（不是输出）是链表的序列化表示，其头节点的值为 2，第二个节点值为 1，第三个节点值为 5 。
链接：https://leetcode-cn.com/problems/next-greater-node-in-linked-list/

We are given a linked list with head as the first node. Let’s number the nodes in the list: node_1, node_2, node_3, … etc.

Each node may have a next larger value: for node_i, next_larger(node_i) is the node_j.val such that j > i, node_j.val > node_i.val, and j is the smallest possible choice. If such a j does not exist, the next larger value is 0.

Return an array of integers answer, where answer[i] = next_larger(node_{i+1}).

Note that in the example inputs (not outputs) below, arrays such as [2,1,5] represent the serialization of a linked list with a head node value of 2, second node value of 1, and third node value of 5.

Example:

Input: [1,7,5,1,9,2,5,1]
Output: [7,9,9,9,0,5,0,0]

思路及代码

类似Leetcode-496、503、739

• 先将linked list转为数组再用单调栈即可

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def nextLargerNodes(self, head: ListNode) -> List[int]:
        stack = []
        l = []
        while head:
            l.append(head.val)
            head = head.next
        ans = [0] * len(l)
        for i in range(len(l)):
            while stack and l[stack[-1]] < l[i]:
                ans[stack.pop()] = l[i]
            stack.append(i)
        return ans