Leetcode - Spiral Matrix

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].

[分析]
思路1：螺旋遍历就是从外到内一圈圈顺时针遍历，记每一圈起始的坐标为xStart, yStart, 每一圈x方向半径为xRadius, y方向半径为yRadius，则下一圈的起始坐标是xStart+1, yStart+1, x和y方向半径分别是xRadius-2，yRadius-2，循环直到圈任一半径为0。实现时要注意corner case，最后一圈只有一行或者一列，因此要break出来否则最后一圈会添加重复元素。
思路2：参考https://leetcode.com/discuss/12228/super-simple-and-easy-to-understand-solution 代码更简洁易写。需要注意的是给定matrix长宽可能不相同，因此外层循环需要判断rowStart <= rowEnd && colStart <= colEnd，缺一不可，而在Spiral Matrix II中由于长宽相等，故判断一个即可。

public class Solution {
    // Method 1
    public List<Integer> spiralOrder1(int[][] matrix) {
        List<Integer> ret = new ArrayList<Integer>();
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) 
            return ret;
        int xStart = 0, yStart = 0;
        int xRadius = matrix[0].length, yRadius = matrix.length;
        while (xRadius > 0 && yRadius > 0) {
            int i = yStart, j = xStart;
            // move left
            int upRightEdge = xStart + xRadius;
            while (j < upRightEdge) {
                ret.add(matrix[i][j++]);
            }
            j--;
            if (yRadius == 1) break;
            // move down
            int downRightEdge = yStart + yRadius;
            while (++i < downRightEdge) {
                ret.add(matrix[i][j]);
            }
            i--;
            if (xRadius == 1) break;
            // move right
            while (--j >= xStart) {
                ret.add(matrix[i][j]);
            }
            j++;
            // move up
            while (--i > yStart) {
                ret.add(matrix[i][j]);
            }
            i++;
            // update info to start next spiral
            xStart++; 
            yStart++;
            xRadius -= 2;
            yRadius -= 2;
        }
        return ret;
    }
    //Method 2
    public List<Integer> spiralOrder(int[][] matrix) {
        List<Integer> ret = new ArrayList<Integer>();
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0)
            return ret;
        int rowStart = 0, rowEnd = matrix.length - 1;
        int colStart = 0, colEnd = matrix[0].length - 1;
        while (rowStart <= rowEnd && colStart <= colEnd) {
            for (int j = colStart; j <= colEnd; j++) {//move right
                ret.add(matrix[rowStart][j]);
            }
            rowStart++;
            for (int i = rowStart; i <= rowEnd; i++) {//move down
                ret.add(matrix[i][colEnd]);
            }
            colEnd--;
            if (rowEnd >= rowStart) {
                for (int j = colEnd; j >= colStart; j--) {//move left
                    ret.add(matrix[rowEnd][j]);
                }
                rowEnd--;
            }
            if (colEnd >= colStart) {
                for (int i = rowEnd; i >= rowStart; i--) {//move up
                    ret.add(matrix[i][colStart]);
                }
                colStart++;
            }
        }
        return ret;
    }
    
}