Spiral Matrix II

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Given positive integer n, generate an n x n matrix filled with elements from 1 to n^2 in spiral order.

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

Solution Approach

Same boundary shrinking approach but write numbers 1 to n^2 in spiral order.

  • Same four-boundary approach as Spiral Matrix.
  • Write num from 1 to n^2 while filling.
Java
class Solution {
    public int[][] generateMatrix(int n) {
        int[][] mat = new int[n][n];
        int top=0, bottom=n-1, left=0, right=n-1, num=1;
        while (top<=bottom && left<=right) {
            for (int c=left;c<=right;c++) mat[top][c]=num++;    top++;
            for (int r=top;r<=bottom;r++) mat[r][right]=num++;  right--;
            if (top<=bottom) { for (int c=right;c>=left;c--) mat[bottom][c]=num++; bottom--; }
            if (left<=right) { for (int r=bottom;r>=top;r--) mat[r][left]=num++; left++; }
        }
        return mat;
    }
}
  • Time Complexity: O(N^2)
  • Space Complexity: O(1) extra