496. Next Greater Element I

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

• 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?

Solution

 func nextGreaterElement(nums1 []int, nums2 []int) []int {
	stack := []int{}
	numsMap := map[int]int{nums2[len(nums2)-1]: -1}
	result := []int{}
	for i := len(nums2) - 1; i >= 0; i-- {
		if len(stack) > 0 {
			switch {
			case nums2[i] < stack[len(stack)-1]:
				numsMap[nums2[i]] = stack[len(stack)-1]
			case nums2[i] > stack[len(stack)-1]:
				for len(stack) > 0 && nums2[i] > stack[len(stack)-1] {
					stack = stack[:len(stack)-1]
				}
				if len(stack) > 0 {
					numsMap[nums2[i]] = stack[len(stack)-1]
				} else {
					numsMap[nums2[i]] = -1
				}
			}
		}
		stack = append(stack, nums2[i])
	}

	for _, v := range nums1 {
		result = append(result, numsMap[v])
	}
	return result
}