Leetcode #645: Set Mismatch
In this guide, we solve Leetcode #645 Set Mismatch 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
You have a set of integers s, which originally contains all the numbers from 1 to n. Unfortunately, due to some error, one of the numbers in s got duplicated to another number in the set, which results in repetition of one number and loss of another number.
Quick Facts
- Difficulty: Easy
- Premium: No
- Tags: Bit Manipulation, Array, Hash Table, Sorting
Intuition
Fast membership checks and value lookups are the heart of this problem, which makes a hash map the natural choice.
By storing what we have already seen (or counts/indexes), we can answer the question in one pass without backtracking.
Approach
Scan the input once, using the map to detect when the condition is satisfied and to update state as you go.
This keeps the solution linear while remaining easy to explain in an interview setting.
Steps:
- Initialize a hash map for seen items or counts.
- Iterate through the input, querying/updating the map.
- Return the first valid result or the final computed value.
Example
Input: nums = [1,2,2,4]
Output: [2,3]
Python Solution
from typing import List
def findErrorNums(nums: List[int]) -> List[int]:
n = len(nums)
seen = set()
dup = -1
for x in nums:
if x in seen:
dup = x
else:
seen.add(x)
missing = next(i for i in range(1, n + 1) if i not in seen)
return [dup, missing]
Complexity
The time complexity is , and the space complexity is , where is the length of the array . The space complexity is , where is the length of the array .
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.