Leetcode 496. Next Greater Element I

You are given two integer arrays nums1 and nums2 both of unique elements, where nums1 is a 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, return -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.

 

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? 

Code (Java)

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

class Solution {     public int[] nextGreaterElement(int[] nums1, int[] nums2) {         if (nums1 == null || nums1.length == 0 || nums2 == null || nums2.length == 0) {             return new int[0];         }                   int[] ans = new int[nums1.length];                   Stack<Integer> monoDescStack = new Stack<>();         // key is the elements in nums2, value is the next greater element         Map<Integer, Integer> map = new HashMap<>();                   for (int num : nums2) {             while (!monoDescStack.isEmpty() && num > monoDescStack.peek()) {                 int top = monoDescStack.pop();                 map.put(top, num);             }                           monoDescStack.push(num);         }                   for (int i = 0; i < nums1.length; i++) {             if (map.containsKey(nums1[i])) {                 ans[i] = map.get(nums1[i]);             } else {                 ans[i] = -1;             }         }                   return ans;     } }