NextGreaterElement.java

package LeetcodePractice;

import java.util.Arrays;
import java.util.Map;
import java.util.Stack;
import java.util.HashMap;

public class NextGreaterElement {
     public static int[] nextGreaterElement(int[] nums1, int[] nums2) {
        Map<Integer, Integer> map = new HashMap<>();
        Stack<Integer> stack = new Stack<>();
        for (int num : nums2) {
            while (!stack.isEmpty() && stack.peek() < num) map.put(stack.pop(), num);
            stack.push(num);
        }
        int[] res = new int[nums1.length];
        for (int i = 0; i < nums1.length; i++) res[i] = map.getOrDefault(nums1[i], -1);
        return res;
    }

    public static void main(String[] args) {
        int[] nums1 = {4, 1, 2};
        int[] nums2 = {1, 3, 4, 2};
        System.out.println(Arrays.toString(nextGreaterElement(nums1, nums2)));

        int[] nums3 = {2, 4};
        int[] nums4 = {1, 2, 3, 4};
        System.out.println(Arrays.toString(nextGreaterElement(nums3, nums4)));
    }
}