59. Spiral Matrix II

Problem Statement:

Given a positive integer n, generate an n x n matrix filled with elements from 1 to n2 in spiral order.

Example 1:
Input: n = 3
Output: [[1,2,3],[8,9,4],[7,6,5]]

Example 2:
Input: n = 1
*Output: *[[1]]

Constraints:

• 1 <= n <= 20

Solution:

Algorithm:

1. Create a matrix of size n x n.
2. Initialize left = 0, right = n - 1, top = 0, bottom = n - 1, num = 1
3. Iterate while left <= right and top <= bottom. a) Iterate from left to right and fill the top row with num. b) Increment top. c) Iterate from top to bottom and fill the right column with num. d) Decrement right. e) Iterate from right to left and fill the bottom row with num. f) Decrement bottom. g) Iterate from bottom to top and fill the left column with num. h) Increment left.
4. Return matrix.

Code:

public class Solution {
    public int[][] generateMatrix(int n){
        int[][] matrix = new int[n][n];
        int left = 0;
        int right = n - 1;
        int top = 0;
        int bottom = n - 1;
        int num = 1;
        while(left <= right && top <= bottom){
            for(int i = left; i <= right; i++){
                matrix[top][i] = num++;
            }
            top++;
            for(int i = top; i <= bottom; i++){
                matrix[i][right] = num++;
            }
            right--;
            for(int i = right; i >= left; i--){
                matrix[bottom][i] = num++;
            }
            bottom--;
            for(int i = bottom; i >= top; i--){
                matrix[i][left] = num++;
            }
            left++;
        }
    }
}

Time Complexity:
O(N^2)

Space Complexity:
O(N^2)