What is DSA and why is it important for interviews?

DSA (Data Structures and Algorithms) forms the foundation of coding interviews at top tech companies. It tests problem-solving skills, coding ability, and understanding of time/space complexity. Companies like Google, Amazon, Meta, and Microsoft heavily focus on DSA in their technical interviews.

How many DSA problems should I solve before interviews?

For FAANG-level interviews, aim to solve 150-200 quality problems covering all major topics. Focus on understanding patterns rather than memorizing solutions. Cover: 30+ Array/String problems, 20+ Tree problems, 15+ Graph problems, 25+ DP problems, and 10+ problems each for Stack, Heap, and Binary Search.

What topics are most important for DSA interviews?

Key DSA topics: Arrays & Strings (Two Sum, Sliding Window), Binary Search, Linked Lists, Trees & BST (traversals, LCA), Graphs (BFS, DFS, shortest path), Dynamic Programming (1D & 2D), Stack & Queue, Heap/Priority Queue, Backtracking, and Greedy Algorithms.

Is InstaMock DSA practice free?

Yes! InstaMock offers 150+ DSA problems for free with instant code execution in Java, Python, and C++. You can run test cases, submit solutions, and get immediate feedback without any payment.

How is InstaMock different from LeetCode?

InstaMock combines DSA practice with AI mock interviews. While you can solve coding problems like LeetCode, you also get AI-powered interview simulations, real-time feedback, and comprehensive preparation for System Design (HLD/LLD) along with DSA.

Number of Closed Islands - Graph Traversal

Number of Closed Islands

Given a grid where 0 represents land and 1 represents water, return the number of closed islands. A closed island is a land component that is completely surrounded by water and does not touch the border. Pattern focus: Component traversal. Count only the components that never reach the boundary.

Input Format

grid = 2D binary matrix

Output Format

number of closed islands

Constraints

1 <= rows * cols <= 10^5
Each cell is 0 or 1, where 0 is land.

Examples

Example 1:

Input:

grid = [[1,1,1,1,1,1,1,0],[1,0,0,0,0,1,1,0],[1,0,1,0,1,1,1,0],[1,0,0,0,0,1,0,1],[1,1,1,1,1,1,1,0]]

Output:

Explanation:

There are two closed islands made of 0s.

Example 2:

Input:

grid = [[0,0,1,0,0],[0,1,1,1,0],[0,1,0,1,0],[0,1,1,1,0],[0,0,0,0,0]]

Output:

Explanation:

Only one land component of 0s is fully enclosed.

Example 3:

Input:

grid = [[1,1,1],[1,1,1],[1,1,1]]

Output:

Explanation:

There is no land at all.