54. Spiral Matrix

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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].

初始化rowBegin = 0, rowEnd = matrix.length - 1, colBegin = 0, colEnd = matrix[0].length - 1。开始按照螺旋方向添加元素，并加减相应的rowBegin, rowEnd, colBegin, colEnd。注意一行或者一列的情况，要加额外判断，代码如下：

public class Solution {    public List<Integer> spiralOrder(int[][] matrix) {        List<Integer> res = new ArrayList<Integer>();        if (matrix.length == 0) {            return res;        }        int rowBegin = 0, rowEnd = matrix.length - 1;        int colBegin = 0, colEnd = matrix[0].length - 1;        while (rowBegin <= rowEnd && colBegin <= colEnd) {            for (int i = colBegin; i <= colEnd; i ++) {                res.add(matrix[rowBegin][i]);            }            rowBegin ++;            for (int i = rowBegin; i <= rowEnd; i ++) {                res.add(matrix[i][colEnd]);            }            colEnd --;            if (rowBegin <= rowEnd) {                for (int i = colEnd; i >= colBegin; i --) {                    res.add(matrix[rowEnd][i]);                }            }            rowEnd --;            if (colBegin <= colEnd) {                for (int i = rowEnd; i >= rowBegin; i --) {                    res.add(matrix[i][colBegin]);                }            }            colBegin ++;        }        return res;    }}

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