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)

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;
    }
}