學資料結構的時候有做過一道類似的題目，不一樣的是當時的二維陣列規定是正方形，而這裡是矩形，然後我就不會了。。 

演算法圖示（4x3矩陣）：

ac程式碼（java）：

import java.util.ArrayList;

public class Solution {
	public Solution() {
		int test[][] = new int[][] { { 1, 2, 3}, { 4, 5, 6 }, { 7, 8, 9}, {10, 11, 12 }};
		System.out.println(printMatrix(test)); //[1, 2, 3, 6, 9, 12, 11, 10, 7, 4]
	}
	
	public ArrayList<Integer> printMatrix(int[][] matrix) {
		ArrayList<Integer> AL = new ArrayList<Integer>();
		int left = 0, top = 0, right = matrix[0].length - 1, bottom = matrix.length - 1;
		while (left <= right && top <= bottom) {
			//頂部從左往右
			for (int i = left; i <= right; i++)
				AL.add(matrix[top][i]);
			//右邊從上往下
			for (int i = top + 1; i <= bottom; i++)
				AL.add(matrix[i][right]);
			//底部從右往左
			if (top != bottom) //當二維陣列只有一行時，避免重複輸出
				for (int i = right -1; i >= left; i--)
					AL.add(matrix[bottom][i]);
			//左邊從下往上
			if (left != right) //當二維陣列只有一列時，避免重複輸出
				for (int i = bottom -1; i > top; i--)
					AL.add(matrix[i][left]);
			left++;
			top++;
			right--;
			bottom--;
		}
		return AL;
	}

	/*
	 * if (Math.sqrt(matrix.length * matrix[0].length) % 2 != 0) AL.add(matrix[(int)
	 * (Math.sqrt(matrix.length) / 2) + 1][(int) (Math.sqrt(matrix.length) / 2) +
	 * 1]);
	 */

	public static void main(String args[]) {
		new Solution();
	}

}