Next Greater Element I

Next Greater Element I is a Easy Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Stack Patterns pattern, in the Monotonic Stack subpattern.

Problem statement

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.

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

Topics and companies

Topics: Array, Hash Table, Stack, Monotonic Stack.

Reported in interviews at Amazon, Microsoft.

How to practise Next Greater Element I on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(N + M), where N = nums2.size() and M = nums1.size(). We process each element of nums2 once for the monotonic stack, and then each element of nums1 once for lookup. time and O(N), where N = nums2.size(). The next_greater map stores at most N entries, and the stack at most N elements. space.

Open Next Greater Element I in the code editor, or read the Next Greater Element I editorial for a worked solution with its approach and complexity analysis.

Where Next Greater Element I sits in the DSA pattern sheet

Problems related to Next Greater Element I

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