今天原本是想做一題困難度 Medium 的，但是週五晚上實在是太累了，做半天做不出來，只好在挑一題簡單的來做．

問題

https://leetcode.com/problems/next-greater-element-i/description/

The next greater element of some element x in an array is the first greater element that is to the right of x in the same array.

You are given two distinct 0-indexed integer arrays nums1 and nums2, where nums1 is a subset of nums2.

For each 0 <= i < nums1.length, find the index j such that nums1[i] == nums2[j] and determine the next greater element of nums2[j] in nums2. If there is no next greater element, then the answer for this query is -1.

Return an array ans of length nums1.length such that ans[i] is the next greater element as described above.

Example 1:
Input: nums1 = [4,1,2], nums2 = [1,3,4,2]
Output: [-1,3,-1]
Explanation: The next greater element for each value of nums1 is as follows:

• 4 is underlined in nums2 = [1,3,4,2]. There is no next greater element, so the answer is -1.
• 1 is underlined in nums2 = [1,3,4,2]. The next greater element is 3.
• 2 is underlined in nums2 = [1,3,4,2]. There is no next greater element, so the answer is -1.

Example 2:
Input: nums1 = [2,4], nums2 = [1,2,3,4]
Output: [3,-1]
Explanation: The next greater element for each value of nums1 is as follows:

• 2 is underlined in nums2 = [1,2,3,4]. The next greater element is 3.
• 4 is underlined in nums2 = [1,2,3,4]. There is no next greater element, so the answer is -1.

Constraints:

• 1 <= nums1.length <= nums2.length <= 1000
• 0 <= nums1[i], nums2[i] <= 104
• All integers in nums1 and nums2 are unique.
• All the integers of nums1 also appear in nums2.

我的答案

簡兒言之，就是先把第二數列中每個元素，右邊相對應比他大的元素找出來配對存到 Hash Table 後，在把第一數列中每個元素利用Hash Table拼湊出答案．

Python

class Solution:
    def nextGreaterElement(self, nums1: List[int], nums2: List[int]) -> List[int]:
        stack = []
        mapping = {}
        for value in nums2:
            while stack and value > stack[-1]:
                mapping[stack[-1]] = value
                stack.pop()
            stack.append(value)

        for value in stack:
            mapping[value] = -1

        ans=[]
        for value in nums1:
            ans.append(mapping[value])
        return ans

Go

func nextGreaterElement(nums1 []int, nums2 []int) []int {
    stack := []int{}
    mapping := map[int]int{}

    for _, value := range nums2 {
        n := len(stack)
        for n > 0 && value > stack[n-1] {
            mapping[stack[n-1]] = value
            stack = stack[0:n-1]
            n--
        }
        stack = append(stack, value)
    }

    for _, value := range stack {
        mapping[value] = -1
    }

    ans := []int{}
    for _, value := range nums1 {
        ans = append(ans, mapping[value])
    }
    return ans
}