[leetcode]59. Spiral Matrix II螺旋遍历矩阵2

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

Example:

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

题意：

给定n， 把1至n ^ 2所有的数，按照螺旋顺序填入方阵。

code

/*
Time: O(n^2)
Space: O(n^2)
*/
class Solution {
    public int[][] generateMatrix(int n) {
        int[][] matrix = new int[n][n];
        if (n == 0) return matrix;
        int beginX = 0, endX = n - 1;
        int beginY = 0, endY = n - 1;
        int num = 1;
        while (true) {
            for (int j = beginX; j <= endX; ++j) matrix[beginY][j] = num++;
            if (++beginY > endY) break;

            for (int i = beginY; i <= endY; ++i) matrix[i][endX] = num++;
            if (beginX > --endX) break;

            for (int j = endX; j >= beginX; --j) matrix[endY][j] = num++;
            if (beginY > --endY) break;

            for (int i = endY; i >= beginY; --i) matrix[i][beginX] = num++;
            if (++beginX > endX) break;
        }
        return matrix;
    }
}