class Solution:
    def nextGreaterElements(self, nums: List[int]) -> List[int]:
        if not nums:
            return []

        ans = [-1 for i in range(len(nums))] # default not found
        stack = []

        # first round
        for i, num in enumerate(nums):
            while stack and nums[stack[-1]] < num:
                idx = stack.pop()
                ans[idx] = num                
            stack.append(i) # push index in stack

        # sencond round, clear stack    
        for i, num in enumerate(nums):
            if not stack:
                break 
            while stack and nums[stack[-1]] < num:
                idx = stack.pop()
                ans[idx] = num                

        return ans