Leetcode #665: Non-decreasing Array
In this guide, we solve Leetcode #665 Non-decreasing Array in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Given an array nums with n integers, your task is to check if it could become non-decreasing by modifying at most one element. We define an array is non-decreasing if nums[i] <= nums[i + 1] holds for every i (0-based) such that (0 <= i <= n - 2).
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Array
Intuition
The constraints allow a direct scan that keeps only the essential state.
By translating the requirements into a clean loop, the logic stays easy to reason about.
Approach
Iterate through the data once, updating the state needed to compute the answer.
Return the final state after the traversal is complete.
Steps:
- Parse the input.
- Iterate and update state.
- Return the computed answer.
Example
Input: nums = [4,2,3]
Output: true
Explanation: You could modify the first 4 to 1 to get a non-decreasing array.
Python Solution
class Solution:
def checkPossibility(self, nums: List[int]) -> bool:
def is_sorted(nums: List[int]) -> bool:
return all(a <= b for a, b in pairwise(nums))
n = len(nums)
for i in range(n - 1):
a, b = nums[i], nums[i + 1]
if a > b:
nums[i] = b
if is_sorted(nums):
return True
nums[i] = nums[i + 1] = a
return is_sorted(nums)
return True
Complexity
The time complexity is O(n). The space complexity is O(1) to O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.