Monotonic stack keeps elements in increasing or decreasing order.

## Problem: next greater element
```python
def next_greater(nums):
    st = []
    res = [-1]*len(nums)
    for i in range(len(nums)):
        while st and nums[i] > nums[st[-1]]:
            j = st.pop()
            res[j] = nums[i]
        st.append(i)
    return res

print(next_greater([2,1,2,4,3]))
```

## Graph
```mermaid
flowchart LR
  A[Stack] --> B[Keep order]
  B --> C[Pop when broken]
```

## Remember
- stack keeps monotonic order
- solves many range problems in O(n)


Use monotonic stacks to solve next greater/smaller problems efficiently in linear time.

Monotonic Stack

Master fundamental data structures and their implementations.

Monotonic Stack

Problem: next greater element

Graph

Remember

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