Problem 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:

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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:

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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.

Follow up:

Could you find an O(nums1.length + nums2.length)solution?

Solution:

traverse nums2

stack to keep traversed element

when encounter an element nums2[i] > stk[-1], means nums2[i] is the next greater element for some elements in the stack

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class Solution:
    def nextGreaterElement(self, nums1: List[int], nums2: List[int]) -> List[int]:
        '''
        walk through nums2
        use a stack to check nums2[i] is greater then stack.top()
        if it's greater, then record it in hashmap
        '''
        stk = deque([])
        mapping = defaultdict()
        
        for i in range(len(nums2)):
            if stk and stk[-1] < nums2[i]:
                while stk and nums2[i] > stk[-1]:
                    top = stk.pop()
                    mapping[top] = nums2[i]
    
            stk.append(nums2[i])
                
        # print(mapping)
        res = []
        for i in range(len(nums1)):
            if nums1[i] not in mapping:
                res.append(-1)
            else:
                res.append(mapping[nums1[i]])
        return res

time complexity: $O()$
space complexity: $O()$
reference:
related problem: