Hola,

The basic naive approach is solving this problem in O(n^2) time complexity, where for every element in array 1 we iterate through every element in array 2 and as soon as we find match then we look for greater element to it's right in array 2 if not present then it is going to be -1.

But we can solve this problem in O(n) using Map and Stack data structures together, the algorithm that going to consider is "monotonic stack".

What is monotonic stack?

Problem: Next Greater Element I

Solution: Watch YouTube video

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

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