Next Greater Element — Core Logic

Scanning left to right, the stack remembers candidates that still need a next greater value to their right.

Try this in Python

def next_greater(nums: list[int]) -> list[int | None]:
    n = len(nums)
    ans: list[int | None] = [None] * n
    st: list[int] = []
    for i in range(n - 1, -1, -1):
        while st and st[-1] <= nums[i]:
            st.pop()
        ans[i] = st[-1] if st else None
        st.append(nums[i])
    return ans


print(next_greater([2, 4, 0, 9, 6]))

Key takeaways

• Reverse scan is one clean way to think “next on the right.”
• Forward-with-indices is equally common in interviews.
• Tie-breaking (<= vs <) changes behavior—state it explicitly.