Leetcode 59: Spiral Matrix II

1. Problem Statement (Simple Explanation)

You’re given a positive integer n.

You must generate an n x n matrix filled with the numbers 1 to n², in spiral order:

• Start from the top-left, go right, then down, then left, then up, and so on, spiraling inward.

2. Examples

Example 1:

Input: n = 3

Output:

[

  [1,2,3],

  [8,9,4],

  [7,6,5]

]

Filled in spiral from 1 to 9.

Example 2:

Input: n = 1

Output:

[

  [1]

]

3. Approach – Boundary Tracking (Same Concept as Spiral Matrix I)

We use four boundaries:

• top – current top row index

• bottom – current bottom row index

• left – current left column index

• right – current right column index

And a counter num from 1 to n*n.

While top <= bottom and left <= right:

1. Fill top row from left to right. Increment top.

2. Fill right column from top to bottom. Decrement right.

3. If top <= bottom, fill bottom row from right to left. Decrement bottom.

4. If left <= right, fill left column from bottom to top. Increment left.

Pseudo-code:

Complexity:

• Time: We fill each cell once → O(n2)

• Space: O(n2) for the matrix (required output), extra variables are O(1)

4. Java code

5. C code

6. C++ code

7. Python code

8. JavaScript code