🚀Day 2. Longest Bounded-Difference Subarray 🧠

The problem can be found at the following link: Question Link

💡 Problem Description:

Given an array of positive integers arr[] and a non-negative integer x, the task is to find the longest sub-array where the absolute difference between any two elements is not greater than x.

If multiple such subarrays exist, return the one that starts at the smallest index.

🔍 Example Walkthrough:

Example 1

Input

arr[] = [8, 4, 2, 6, 7]
x = 4

Output

[4, 2, 6]

Explanation

• The sub-array [4, 2, 6] has no pair of elements with absolute difference greater than 4.

• Other subarrays either don't meet the condition or are shorter.

Example 2

Input

arr[] = [15, 10, 1, 2, 4, 7, 2]
x = 5

Output

[2, 4, 7, 2]

Explanation

• The sub-array described by indexes [3..6], i.e. [2, 4, 7, 2] contains no such difference of two elements which is greater than 5.

Constraints

• $1 \leq N \leq 10^5$

• $1 \leq arr[i] \leq 10^9$

• $0 \leq x \leq 10^9$

🎯 My Approach:

Sliding Window with Deque

This approach uses two deques to dynamically maintain the minimum and maximum elements within the sliding window.

• minQ tracks the minimum values in the current window.

• maxQ tracks the maximum values in the current window.

• If maxQ.front() - minQ.front() exceeds x, we shrink the window from the left.

• At every step, we check if the current window length is the longest valid window seen so far.

Steps:

1. Initialize start pointer and two deques: minQ (for minimum) and maxQ (for maximum).

2. Expand the window by adding the current element to both deques.

3. If the difference between max and min exceeds x, shrink the window by incrementing start.

4. Track the longest valid window and its starting position.

5. Return the subarray corresponding to the longest valid window.

🕒 Time and Auxiliary Space Complexity

Complexity	Analysis
Time Complexity	O(N) - Each element is pushed and popped from the deque at most once.
Space Complexity	O(N) - In the worst case, both deques can hold all indices.

📝 Solution Code

Code (C++)

class Solution {
public:
    vector<int> longestSubarray(vector<int>& arr, int x) {
        deque<int> minQ, maxQ;
        int n = arr.size(), start = 0, resStart = 0, resEnd = 0;
        for (int end = 0; end < n; end++) {
            while (!minQ.empty() && arr[minQ.back()] > arr[end]) minQ.pop_back();
            while (!maxQ.empty() && arr[maxQ.back()] < arr[end]) maxQ.pop_back();
            minQ.push_back(end), maxQ.push_back(end);
            while (arr[maxQ.front()] - arr[minQ.front()] > x) {
                if (minQ.front() == start) minQ.pop_front();
                if (maxQ.front() == start) maxQ.pop_front();
                start++;
            }
            if (end - start > resEnd - resStart) resStart = start, resEnd = end;
        }
        return vector<int>(arr.begin() + resStart, arr.begin() + resEnd + 1);
    }
};

⚡ Alternative Approaches

2️⃣ Using Ordered Set (O(N log N) Time, O(N) Space)

Key Idea

• Use a multiset to dynamically maintain the window elements.

• Get the min and max in constant time using *begin() and *rbegin().

• Shrink the window if the max-min difference exceeds x.

class Solution {
public:
    vector<int> longestSubarray(vector<int>& arr, int x) {
        multiset<int> window;
        int start = 0, resStart = 0, resLen = 0;
        for (int end = 0; end < arr.size(); end++) {
            window.insert(arr[end]);
            while (*window.rbegin() - *window.begin() > x) window.erase(window.find(arr[start++]));
            if (end - start + 1 > resLen) resStart = start, resLen = end - start + 1;
        }
        return vector<int>(arr.begin() + resStart, arr.begin() + resStart + resLen);
    }
};

3️⃣ Brute Force (O(N²) Time, O(1) Space)

Key Idea

• For every subarray arr[i..j], check the max and min values and verify the condition.

• This is only feasible for small arrays.

class Solution {
public:
    vector<int> longestSubarray(vector<int>& arr, int x) {
        int n = arr.size(), maxLen = 0, resStart = 0;
        for (int i = 0; i < n; i++) {
            int minVal = arr[i], maxVal = arr[i];
            for (int j = i; j < n; j++) {
                minVal = min(minVal, arr[j]), maxVal = max(maxVal, arr[j]);
                if (maxVal - minVal > x) break;
                if (j - i + 1 > maxLen) resStart = i, maxLen = j - i + 1;
            }
        }
        return vector<int>(arr.begin() + resStart, arr.begin() + resStart + maxLen);
    }
};

📊 Comparison of Approaches

Approach	⏱️ Time Complexity	🗂️ Space Complexity	✅ Pros	⚠️ Cons
Sliding Window + Deque (Optimal)	🟢 O(N)	🟢 O(N)	Fastest for all cases	Slightly complex
Ordered Set (Multiset)	🟡 O(N log N)	🟡 O(N)	Elegant window handling	Slower than deque
Brute Force	🔴 O(N²)	🟢 O(1)	Simple to implement	Very slow for large arrays

💡 Best Choice?

• ✅ For optimal performance: Sliding Window + Monotonic Deque (O(N) time, O(N) space).

• ✅ For simpler code: Ordered Set is easier than deques.

• ✅ For small inputs: Brute Force is acceptable for $N \leq 100$.

Code (Java)

class Solution {
    public ArrayList<Integer> longestSubarray(int[] arr, int x) {
        Deque<Integer> minQ = new ArrayDeque<>(), maxQ = new ArrayDeque<>();
        int n = arr.length, start = 0, resStart = 0, resLen = 0;
        for (int end = 0; end < n; end++) {
            while (!minQ.isEmpty() && arr[minQ.peekLast()] > arr[end]) minQ.pollLast();
            while (!maxQ.isEmpty() && arr[maxQ.peekLast()] < arr[end]) maxQ.pollLast();
            minQ.addLast(end);
            maxQ.addLast(end);
            while (arr[maxQ.peekFirst()] - arr[minQ.peekFirst()] > x) {
                if (minQ.peekFirst() == start) minQ.pollFirst();
                if (maxQ.peekFirst() == start) maxQ.pollFirst();
                start++;
            }
            if (end - start + 1 > resLen) {
                resStart = start;
                resLen = end - start + 1;
            }
        }
        ArrayList<Integer> res = new ArrayList<>();
        for (int i = resStart; i < resStart + resLen; i++) res.add(arr[i]);
        return res;
    }
}

Code (Python)

from collections import deque

class Solution:
    def longestSubarray(self, arr, x):
        minQ, maxQ, start, resStart, resEnd = deque(), deque(), 0, 0, 0
        for end in range(len(arr)):
            while minQ and arr[minQ[-1]] > arr[end]: minQ.pop()
            while maxQ and arr[maxQ[-1]] < arr[end]: maxQ.pop()
            minQ.append(end), maxQ.append(end)
            while arr[maxQ[0]] - arr[minQ[0]] > x:
                if minQ[0] == start: minQ.popleft()
                if maxQ[0] == start: maxQ.popleft()
                start += 1
            if end - start > resEnd - resStart: resStart, resEnd = start, end
        return arr[resStart:resEnd + 1]

🎯 Contribution and Support:

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