Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

For example,
Given the following matrix:

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

You should return[1,2,3,6,9,8,7,4,5].

https://oj.leetcode.com/problems/spiral-matrix/

思路：这题目不难理解，就是注意边界情况， 推荐用 上下左右4个bar的方法，简单不易出错。

import java.util.ArrayList;public class Solution {    public ArrayList<Integer> spiralOrder(int[][] matrix) {        ArrayList<Integer> res = new ArrayList<Integer>();        if (matrix == null || matrix.length < 1 || matrix[0].length < 1)            return res;        int left = 0;        int right = matrix[0].length - 1;        int top = 0;        int bottom = matrix.length - 1;        while (left <= right && top <= bottom) {            int i;            for (i = left; i <= right; i++)                res.add(matrix[top][i]);            for (i = top + 1; i <= bottom; i++)                res.add(matrix[i][right]);            if (top < bottom)//corner case, no need back to left if top==bottom                for (i = right - 1; i >= left; i--)                    res.add(matrix[bottom][i]);            if (left < right)//corner case, no need back up if left==right                for (i = bottom - 1; i >= top + 1; i--)                    res.add(matrix[i][left]);            top++;            bottom--;            left++;            right--;        }        return res;    }    public static void main(String[] args) {        int[][] matrix = new int[][] { { 1,2 }, { 5,6 }, { 9,10 } };        System.out.println(new Solution().spiralOrder(matrix));    }}