Day 1 - Longest Increasing Subsequence.md

Difficulty	Source	Tags
Medium	160 Days of Problem Solving	Dynamic Programming Binary Search

🚀 Day 1. Longest Increasing Subsequence 🧠

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

💡 Problem Description:

Given an array arr[] of non-negative integers, the task is to find the length of the Longest Strictly Increasing Subsequence (LIS).

A subsequence is strictly increasing if each element in the subsequence is strictly less than the next element.

🔍 Example Walkthrough:

Example 1:

Input:

arr[] = [5, 8, 3, 7, 9, 1]

Output:

3

Explanation:

The longest strictly increasing subsequence could be [5, 7, 9], which has a length of 3.

Example 2:

Input:

arr[] = [0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15]

Output:

6

Explanation:

One of the possible longest strictly increasing subsequences is [0, 2, 6, 9, 13, 15], which has a length of 6.

Example 3:

Input:

arr[] = [3, 10, 2, 1, 20]

Output:

3

Explanation:

The longest strictly increasing subsequence could be [3, 10, 20], which has a length of 3.

Constraints:

• $(1 \leq arr.size() \leq 10^3)$
• $(0 \leq arr[i] \leq 10^6)$

🎯 My Approach:

1️⃣ Optimized Binary Search Approach

Algorithm Steps:

1. Iterate through each element of the array.
2. Maintain a list ans which tracks the smallest possible tail for increasing subsequences of different lengths.
3. For each element, find its correct position in ans using lower_bound (binary search).
4. If the element can extend the current LIS, append it to ans, else replace the element at the correct position in ans to keep ans optimized.
5. The size of ans at the end represents the length of the longest increasing subsequence.

Step-by-step Process

1. Initialize an empty list lis.
2. For each element num in the array:
   • Use binary search (lower_bound) to find the position where num can replace or extend the lis list.
   • If num is larger than all elements in lis, append it.
   • Otherwise, replace the existing element at the correct position with num, maintaining the smallest possible value for subsequences of that length.
3. Return the size of lis, which is the length of the Longest Increasing Subsequence.

🕒 Time and Auxiliary Space Complexity

• Expected Time Complexity: O(N log N), where N is the size of the array, as we perform binary search for each element.
• Expected Auxiliary Space Complexity: O(N), for storing the ans array.

📝 Solution Code

Code (C++)

class Solution {
public:
    int lis(std::vector<int>& arr) {
        std::vector<int> ans;
        for (int num : arr) {
            auto it = std::lower_bound(ans.begin(), ans.end(), num);
            if (it == ans.end()) ans.push_back(num);
            else *it = num;
        }
        return ans.size();
    }
};

⚡ Alternative Approaches

2️⃣ Dynamic Programming Approach (O(N²) Time, O(N) Space)

Algorithm Steps:

1. Use an array dp where dp[i] stores the length of the LIS ending at index i.
2. For each element at index i, check all previous elements j.
3. If arr[j] < arr[i], update dp[i] = max(dp[i], dp[j] + 1).
4. Return the maximum value in dp.

class Solution {
public:
    int lis(vector<int>& arr) {
        int n = arr.size();
        vector<int> dp(n, 1);
        int maxLen = 1;
        for (int i = 1; i < n; i++) {
            for (int j = 0; j < i; j++) {
                if (arr[i] > arr[j]) dp[i] = max(dp[i], dp[j] + 1);
            }
            maxLen = max(maxLen, dp[i]);
        }
        return maxLen;
    }
};

3️⃣ Segment Tree with Coordinate Compression Approach (O(N log N) Time, O(N) Space)

Algorithm Steps:

1. Coordinate compress the array values to reduce range size.
2. Use a segment tree to store the longest subsequence length ending at each value.
3. For each element, query the segment tree for the best LIS ending at a smaller value.
4. Update the segment tree to reflect the LIS ending at the current element.

class Solution {
public:
    int lis(vector<int>& arr) {
        int n = arr.size();
        unordered_map<int, int> comp;
        vector<int> sortedArr(arr.begin(), arr.end());
        sort(sortedArr.begin(), sortedArr.end());
        for (int i = 0; i < n; i++) comp[sortedArr[i]] = i + 1;

        vector<int> segTree(n + 1, 0);

        auto query = [&](int idx) {
            int best = 0;
            while (idx > 0) {
                best = max(best, segTree[idx]);
                idx -= idx & -idx;
            }
            return best;
        };

        auto update = [&](int idx, int val) {
            while (idx <= n) {
                segTree[idx] = max(segTree[idx], val);
                idx += idx & -idx;
            }
        };

        int res = 0;
        for (int num : arr) {
            int index = comp[num];
            int best = query(index - 1) + 1;
            update(index, best);
            res = max(res, best);
        }
        return res;
    }
};

📊 Comparison of Approaches

Approach	⏱️ Time Complexity	🗂️ Space Complexity	✅ Pros	⚠️ Cons
Binary Search + DP Array	🟢 O(N log N)	🟡 O(N)	Fast and optimal for large N	No LIS sequence reconstruction
Dynamic Programming (DP)	🟡 O(N²)	🟡 O(N)	Simple to understand	Slow for large arrays
Segment Tree	🟢 O(N log N)	🟡 O(N)	Handles dynamic LIS queries	Complex implementation

💡 Best Choice?

• ✅ For large arrays: Use Binary Search (O(N log N)) for optimal performance.
• ✅ For simplicity: Use Dynamic Programming (O(N²)) for small arrays (N ≤ 1000).
• ✅ For dynamic updates: Use Segment Tree (O(N log N)), especially if array values need frequent updates.

Code (Java)

class Solution {
    public int lis(int[] arr) {
        ArrayList<Integer> ans = new ArrayList<>();
        for (int num : arr) {
            int idx = Collections.binarySearch(ans, num);
            if (idx < 0) idx = -idx - 1;
            if (idx == ans.size()) ans.add(num);
            else ans.set(idx, num);
        }
        return ans.size();
    }
}

Code (Python)

import bisect

class Solution:
    def lis(self, arr):
        ans = []
        for num in arr:
            idx = bisect.bisect_left(ans, num)
            if idx == len(ans):
                ans.append(num)
            else:
                ans[idx] = num
        return len(ans)

🎯 Contribution and Support:

For discussions, questions, or doubts related to this solution, feel free to connect on LinkedIn: Any Questions. Let’s make this learning journey more collaborative!

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