【leetcode】-496. Next Greater Element I 下一个更大元素I

题目

You are given two arrays (without duplicates) nums1 and nums2 where nums1’s elements are subset of nums2. Find all the next greater numbers for nums1’s elements in the corresponding places of nums2.

The Next Greater Number of a number x in nums1 is the first greater number to its right in nums2. If it does not exist, output -1 for this number.

Example 1:
Input: nums1 = [4,1,2], nums2 = [1,3,4,2].
Output: [-1,3,-1]

Explanation:
For number 4 in the first array, you cannot find the next greater number for it in the second array, so output -1.
For number 1 in the first array, the next greater number for it in the second array is 3.
For number 2 in the first array, there is no next greater number for it in the second array, so output -1.

Example 2:
Input: nums1 = [2,4], nums2 = [1,2,3,4].
Output: [3,-1]

Explanation:
For number 2 in the first array, the next greater number for it in the second array is 3.
For number 4 in the first array, there is no next greater number for it in the second array, so output -1.

Note:
All elements in nums1 and nums2 are unique.
The length of both nums1 and nums2 would not exceed 1000.

暴力法

题目要求查找nums1的元素在nums2中位置的右边第一个比该数大的数，那么根据题意直接遍历nums1然后逐个判断nums2右边的元素。

python 代码

class Solution:
    def nextGreaterElement(self, nums1: List[int], nums2: List[int]) -> List[int]:
        if not nums1:
            return []
        res = []
        for num in nums1:
            idx = nums2.index(num)
            if idx == len(nums2)-1:
                res.append(-1)
            else:
                for i in range(idx+1,len(nums2)):
                    if nums2[i]>num:
                        res.append(nums2[i])
                        break
                    elif i+1 == len(nums2):
                        res.append(-1)
        return res

栈

利用栈结构将nums2第一个较大元素的比较结果记录下来，然后对应nums1中元素进行查找.

python 代码

class Solution:
    def nextGreaterElement(self, nums1: List[int], nums2: List[int]) -> List[int]:
        if not nums1:
            return []
        dic,s = {},[]
        for num in nums2:
            while s and s[-1]