Leetcode #581: Shortest Unsorted Continuous Subarray
In this guide, we solve Leetcode #581 Shortest Unsorted Continuous Subarray 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 integer array nums, you need to find one continuous subarray such that if you only sort this subarray in non-decreasing order, then the whole array will be sorted in non-decreasing order. Return the shortest such subarray and output its length.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Stack, Greedy, Array, Two Pointers, Sorting, Monotonic Stack
Intuition
The constraints hint that we can reason about two ends of the data at once, which is perfect for a two-pointer scan.
Moving one pointer at a time keeps the invariant intact and avoids nested loops.
Approach
Place pointers at the left and right ends and move them based on the comparison or target condition.
This yields a clean linear pass after any required sorting.
Steps:
- Set left and right pointers.
- Move a pointer based on the condition.
- Update the best answer while scanning.
Example
Input: nums = [2,6,4,8,10,9,15]
Output: 5
Explanation: You need to sort [6, 4, 8, 10, 9] in ascending order to make the whole array sorted in ascending order.
Python Solution
class Solution:
def findUnsortedSubarray(self, nums: List[int]) -> int:
arr = sorted(nums)
l, r = 0, len(nums) - 1
while l <= r and nums[l] == arr[l]:
l += 1
while l <= r and nums[r] == arr[r]:
r -= 1
return r - l + 1
Complexity
The time complexity is , and the space complexity is . The space complexity is .
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.