LeetCode 219: Contains Duplicate II — Python Solution

Solve LeetCode 219: Contains Duplicate II in Python with a last-index tracking approach. The key is to make the state invariant explicit, so the implementation and complexity follow naturally.

This guide paraphrases the task and does not reproduce LeetCode’s prompt. Use the official page for the complete statement, examples, constraints, and submission runner.

What the problem is testing

Record the most recent index of each value and test whether the distance to the current occurrence is within k.

Algorithm

1. Record the most recent index of each value and test whether the distance to the current occurrence is within k.
2. Maintain this invariant: The map holds the closest possible prior occurrence for every value.
3. Continue until every input item or reachable state has been resolved, then return the accumulated result.

Python solution

from collections import Counter, defaultdict, deque, OrderedDict
import random

class Solution:
    def containsNearbyDuplicate(self, nums, k):
        last = {}
        for i, value in enumerate(nums):
            if value in last and i - last[value] <= k: return True
            last[value] = i
        return False

Why this is correct

The proof follows the maintained state: The map holds the closest possible prior occurrence for every value. Each iteration preserves that claim while permanently resolving at least one position, node, interval, or search state. When the loop or recursion ends, every candidate required by the problem has therefore been included or ruled out, so the returned value is correct.

Complexity

O(n) time and O(n) space. The stated auxiliary space excludes the returned output unless the output is the data structure being built.

Edge cases

k equal to zero can never match distinct indices.

Tested reference code

This implementation is included in the site’s downloadable 100-solution Python library. The complete suite compiles every solution and runs a behavioral assertion for every problem before publication.

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