59. Spiral Matrix II

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

For example,
Given n = 3,

You should return the following matrix:

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

Solution:

The idea is from the origin, move right, down, left, and up while inserting an increment number to the corresponding position.

We should be careful with the boundary.



Code:

public class Solution {
    public int[][] generateMatrix(int n) {
        if (n == 0) {
            return new int[0][0];
        }
        int[][] result = new int[n][n];
        int num = 1;
        int left = 0;
        int right = n - 1;
        int top = 0;
        int bottom = n - 1;
        while (num <= n * n) {
            for (int j = left; j <= right; j++) {
                if (result[top][j] != 0) {
                    break;
                }
                result[top][j] = num++;
            }
            top++;
            for (int i = top; i <= bottom; i++) {
                if (result[i][right] != 0) {
                    break;
                }
                result[i][right] = num++;
            }
            right--;
            for (int j = right; j >= left; j--) {
                if (result[bottom][j] != 0) {
                    break;
                }
                result[bottom][j] = num++;
            }
            bottom--;
            for (int i = bottom; i >= top; i--) {
                if (result[i][left] != 0) {
                    break;
                }
                result[i][left] = num++;
            }
            left++;
        }
        return result;
    }
}