Practice/Google/Leetcode 251. Flatten 2D Vector

Leetcode 251. Flatten 2D Vector

CodingMust

Problem

Design and implement an iterator class that flattens a two-dimensional vector of integers. Your iterator should support two operations:

• next(): Returns the next integer in the flattened sequence
• hasNext(): Returns true if there are more elements to iterate over, false otherwise

The iterator should traverse the 2D vector in row-major order (left to right, top to bottom). Some of the inner vectors may be empty, and your implementation should handle this gracefully without errors.

Requirements

1. Implement a Vector2D class that takes a 2D vector as input in its constructor
2. The next() method should return the next integer in the sequence
3. The hasNext() method should return a boolean indicating if more elements exist
4. Handle empty inner vectors automatically without returning invalid data
5. Both operations should run in amortized O(1) time complexity

Constraints

• The outer vector can contain 0 to 500 inner vectors
• Each inner vector can contain 0 to 500 integers
• Integer values range from -10^9 to 10^9
• next() will only be called when hasNext() returns true
• Your solution should use O(1) space beyond the input storage

Examples

Example 1:

Input: vec = [[1, 2], [3], [4, 5, 6]] Operations: hasNext(), next(), next(), hasNext(), next(), next(), next(), hasNext() Output: [true, 1, 2, true, 3, 4, 5, false] Explanation: The iterator flattens the 2D vector into [1, 2, 3, 4, 5, 6] and iterates through it sequentially.

Example 2:

Input: vec = [[], [7], [], [8, 9], []] Operations: next(), next(), next(), hasNext() Output: [7, 8, 9, false] Explanation: Empty vectors are automatically skipped. The effective sequence is [7, 8, 9].

Example 3:

Input: vec = [[]] Operations: hasNext() Output: [false] Explanation: When all inner vectors are empty, hasNext immediately returns false.