Next Greater Element I

Question

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:

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

Tags

• Array
• Stack

Thought

Basic idea is to use the hash map to store the next greater number when looping through the array. But the implementation approach may vary:

• With stack
• Without stack

The implementation detail is below.

Code

without stack

class Solution(object):
    def nextGreaterElement(self, findNums, nums):
        """
        :type findNums: List[int]
        :type nums: List[int]
        :rtype: List[int]
        """
        nextDict = dict()
        queue = []
        for num in nums:
            nextDict[num] = -1
            newQueue = []
            for current in queue:
                if num > current:
                    nextDict[current] = num
                else:
                    newQueue.append(current)
            queue = newQueue + [num]

        ans = []
        for num in findNums:
            ans.append(nextDict[num])
        return ans

with stack Reference: http://bookshadow.com/weblog/2017/02/05/leetcode-next-greater-element-i/

class Solution(object):
    def nextGreaterElement(self, findNums, nums):
        """
        :type findNums: List[int]
        :type nums: List[int]
        :rtype: List[int]
        """
        dmap = {}
        stack = []
        for n in nums:
            while stack and stack[-1] < n:
                dmap[stack.pop()] = n
            stack.append(n)
        return [dmap.get(n, -1) for n in findNums]