Flatten 2D Vector

Flatten 2D Vector - Problem

Design an iterator to flatten a 2D vector. It should support the next and hasNext operations.

Implement the Vector2D class:

Vector2D(int[][] vec) initializes the object with the 2D vector vec.
next() returns the next element from the 2D vector and moves the pointer one step forward. You may assume that all the calls to next are valid.
hasNext() returns true if there are still some elements in the vector, and false otherwise.

Input & Output

Example 1 — Basic 2D Vector

$ Input: vec = [[1,2],[3],[4]], operations = ["Vector2D","next","next","next","hasNext","hasNext","next","hasNext"]

› Output: [null,1,2,3,true,true,4,false]

💡 Note: Vector2D iterator flattens [[1,2],[3],[4]]. next() returns 1,2,3,4 in sequence. hasNext() checks if more elements exist.

Example 2 — Empty Rows

$ Input: vec = [[1,2],[],[3]], operations = ["Vector2D","next","next","hasNext","next","hasNext"]

› Output: [null,1,2,true,3,false]

💡 Note: Empty middle row is skipped automatically. Elements 1,2,3 are returned in order despite empty row.

Example 3 — Single Row

$ Input: vec = [[1,2,3,4,5]], operations = ["Vector2D","hasNext","next","hasNext"]

› Output: [null,true,1,true]

💡 Note: Single row with multiple elements. hasNext() returns true, next() returns 1, still has more elements.

Constraints

1 ≤ vec.length ≤ 200
0 ≤ vec[i].length ≤ 500
-500 ≤ vec[i][j] ≤ 500
At most 10⁵ calls to next and hasNext

Visualization

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Asked in

G Google 15 f Facebook 12 a Amazon 8 M Microsoft 6

Design an iterator for a 2D vector using two pointers to track row and column positions. The key insight is to automatically skip empty rows and maintain O(1) space complexity. Best approach uses row/column pointers with O(1) time per operation and O(1) space.

Common Approaches

✓ Binary Search

⏱️ Time: N/A Space: N/A

Two Pointers (Row & Column)

⏱️ Time: O(1) Space: O(1)

Maintain row and column pointers to navigate the 2D vector. Skip empty rows automatically and advance to next row when current row is exhausted.

Flatten All at Once

⏱️ Time: O(n) Space: O(n)

Store all elements from the 2D vector in a single list during construction. Use an index pointer to track current position for next() and hasNext() operations.

Sliding Window

⏱️ Time: N/A Space: N/A

Algorithm Steps — Algorithm Steps

Code -

solution.c — C

Time & Space Complexity

Time Complexity

⏱️

✓ Linear Growth

Space Complexity

✓ Linear Space

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