Leetcode Problem 59 Spiral Matrix II.txt

59. Spiral Matrix II
Given a positive integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

Example:

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

Hint to solve: Use 4 pointers for row_min, row_max, col_min, col_max
Loop into 4 directions to fill up the matrix.

public class Solution 
{
    public int[][] GenerateMatrix(int n) 
    {
        //Initialize the result matrix with all zeros.
        int[][] result = new int[n][];
        for(int i = 0; i<n; ++i)
        {
            result[i] = new int[n];
        }
        
        int row_min = 0;
        int row_max = n-1;
        int col_min = 0;
        int col_max = n-1;
        
        int counter = 0;
        while(counter != n*n)
        {
            //Fill the matrix from left to right
            for(int i = row_min; i<= col_max; ++i)
            {
                counter += 1;
                result[row_min][i] = counter;
            }
            row_min += 1;
            
            //Fill the matrix from top to bottom
            for(int i = row_min; i<= row_max; ++i)
            {
                counter += 1;
                result[i][row_max] = counter;
            }
            col_max -= 1;
            
            //Fill the matrix from right to left
            for(int i = col_max; i>= col_min; --i)
            {
                counter += 1;
                result[row_max][i] = counter;
            }
            row_max -= 1;
            
            //Fill the matrix from bottom to top
            for(int i = row_max; i>= row_min; --i)
            {
                counter += 1;
                result[i][col_min] = counter;
            }
            col_min += 1;
        }
        
        return result;
    }
}