Next Greater Element I — Monotonic Stack with HashMap

# Python
def nextGreaterElement(nums1, nums2):
    nge = {}  # val -> next greater in nums2
    stack = []
    for x in nums2:
        while stack and stack[-1] < x:
            nge[stack.pop()] = x
        stack.append(x)
    return [nge.get(x, -1) for x in nums1]

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