Monotonic Stack & Queue

def next_greater_elements(nums: list[int]) -> list[int]:
    n = len(nums)
    res = [-1] * n
    stack = []  # Stores indices of unresolved elements
    
    for i in range(n):
        # Pop all elements smaller than current element nums[i]
        while stack and nums[i] > nums[stack[-1]]:
            popped_idx = stack.pop()
            res[popped_idx] = nums[i]  # nums[i] is the next greater element!
            
        stack.append(i)
        
    return res