59. Spiral Matrix II.java

/*
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
*/
class Solution {
    public int[][] generateMatrix(int n) {
        int[][] result = new int[n][n];
        int cnt = 1;
        for (int layer = 0; layer < (n + 1) / 2; layer++) {
            // direction 1 - traverse from left to right
            for (int ptr = layer; ptr < n - layer; ptr++) {
                result[layer][ptr] = cnt++;
            }
            // direction 2 - traverse from top to bottom
            for (int ptr = layer + 1; ptr < n - layer; ptr++) {
                result[ptr][n - layer - 1] = cnt++;
            }
            // direction 3 - traverse from right to left
            for (int ptr = layer + 1; ptr < n - layer; ptr++) {
                result[n - layer - 1][n - ptr - 1] = cnt++;
            }
            // direction 4 - traverse from bottom to top
            for (int ptr = layer + 1; ptr < n - layer - 1; ptr++) {
                result[n - ptr - 1][layer] = cnt++;
            }
        }
        return result;
    }
}