1299. Replace Elements with Greatest Element on Right Side

#Problem

Given an array arr, replace every element in that array with the greatest element among the elements to its right, and replace the last element with -1.

After doing so, return the array.

https://leetcode.cn/problems/replace-elements-with-greatest-element-on-right-side/

Example 1:

Input: arr = [17,18,5,4,6,1]
Output: [18,6,6,6,1,-1]
Explanation:

• index 0 –> the greatest element to the right of index 0 is index 1 (18).
• index 1 –> the greatest element to the right of index 1 is index 4 (6).
• index 2 –> the greatest element to the right of index 2 is index 4 (6).
• index 3 –> the greatest element to the right of index 3 is index 4 (6).
• index 4 –> the greatest element to the right of index 4 is index 5 (1).
• index 5 –> there are no elements to the right of index 5, so we put -1.

Example 2:

Input: arr = [400]
Output: [-1]
Explanation: There are no elements to the right of index 0.

Constraints:

• 1 <= arr.length <= 10⁴
• 1 <= arr[i] <= 10⁵

#Test Cases

class Solution:
    def replaceElements(self, arr: List[int]) -> List[int]:

import pytest

from solution import Solution


@pytest.mark.parametrize('arr, expected', [
    ([17,18,5,4,6,1], [18,6,6,6,1,-1]),
    ([400], [-1]),
])
@pytest.mark.parametrize('sol', [Solution()])
def test_solution(sol, arr, expected):
    assert sol.replaceElements(arr.copy()) == expected

#Thoughts

可以直接 in-place 修改。从右向左扫描 arr，记录见到的最大的数字，并替换到扫描到的位置。

时间复杂度 O(n)，空间复杂度 O(1)。

#Code

class Solution:
    def replaceElements(self, arr: list[int]) -> list[int]:
        greatest = -1
        for i in range(len(arr) - 1, -1, -1):
            tmp = arr[i]
            arr[i] = greatest
            if tmp > greatest:
                greatest = tmp

        return arr