Contains Duplicate II Problem & Solution

Given an integer array nums and an integer k, return true if there are two distinct indices i and j in the array such that nums[i] == nums[j] and abs(i - j) <= k.

See the contains duplicate ii problem on LeetCode.

C++ Solution

#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")

static const int _=[](){ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();

class Solution {
  bool containsNearbyDuplicate(vector<int>& nums, int k) {

    // Represents the most recent position we saw for each number.
    unordered_map<int, int> js;
    for (int i = 0; i < nums.size(); ++i) {
      if (js.find(nums[i]) != js.end() &&
          abs(i - js[nums[i]]) <= k) {
        return true;

      // Keeps track of the last position we saw for `nums[i]`.
      js[nums[i]] = i;

    return false;

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