Next Greater Element IV — LeetCode 2454 Python Solution

The problem

You are given a 0-indexed array of non-negative integers nums. For each integer in nums, you must find its respective second greater integer.

Example

Input

nums = [2,4,0,9,6]

Output

[9,6,6,-1,-1]

Explanation

0th index: 4 is the first integer greater than 2, and 9 is the second integer greater than 2, to the right of 2.

Python solution

class Solution:
    def secondGreaterElement(self, nums: List[int]) -> List[int]:
        arr = [(x, i) for i, x in enumerate(nums)]
        arr.sort(key=lambda x: -x[0])
        sl = SortedList()
        n = len(nums)
        ans = [-1] * n
        for _, i in arr:
            j = sl.bisect_right(i)
            if j + 1 < len(sl):
                ans[i] = nums[sl[j + 1]]
            sl.add(i)
        return ans

Complexity

Pattern: Heap / Priority Queue

Keep only the best k elements, or always pull the smallest, in log time. LeetCode 2454. Next Greater Element IV is filed here because LeetCode tags it Heap (Priority Queue), which is the vocabulary this hub collects.

The heap / priority queue guide has the Python template for the pattern and the 163 LeetCode problems that use it.

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