1019. Next Greater Node In Linked List

题目

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 1:

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

Example 2:

Input: [2,7,4,3,5]
Output: [7,0,5,5,0]

Example 3:

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

Note:

• 1 <= node.val <= 10^9 for each node in the linked list.
• The given list has length in the range [0, 10000].

题目大意

给出一个链表，要求找出每个结点后面比该结点值大的第一个结点，如果找不到这个结点，则输出 0 。

解题思路

这一题和第 739 题、第 496 题、第 503 题类似。也有 2 种解题方法。先把链表中的数字存到数组中，整道题的思路就和第 739 题完全一致了。普通做法就是 2 层循环。优化的做法就是用单调栈，维护一个单调递减的栈即可。

代码

package leetcode

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */

// 解法一 单调栈
func nextLargerNodes(head *ListNode) []int {
	type node struct {
		index, val int
	}
	var monoStack []node
	var res []int
	for head != nil {
		for len(monoStack) > 0 && monoStack[len(monoStack)-1].val < head.Val {
			res[monoStack[len(monoStack)-1].index] = head.Val
			monoStack = monoStack[:len(monoStack)-1]
		}
		monoStack = append(monoStack, node{len(res), head.Val})
		res = append(res, 0)
		head = head.Next
	}
	return res
}