LeetCode 54. Spiral Matrix（螺旋矩阵）

原题网址：https://leetcode.com/problems/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].

方法：递归。

public class Solution {
    private void spiral(int[][] matrix, int left, int top, int right, int bottom, List spirals) {
        if (left > right || top > bottom) return;
        else if (left == right && top == bottom) {
            spirals.add(matrix[top][left]);
            return;
        } else if (left == right) {
            for(int i=top; i<=bottom; i++) {
                spirals.add(matrix[i][left]);
            }
            return;
        } else if (top == bottom) {
            for(int i=left; i<=right; i++) {
                spirals.add(matrix[top][i]);
            }
            return;
        } else {
            for(int j=left; jleft; j--) spirals.add(matrix[bottom][j]);
            for(int i=bottom; i>top; i--) spirals.add(matrix[i][left]);
            spiral(matrix, left+1, top+1, right-1, bottom-1, spirals);
        }
    }
    public List spiralOrder(int[][] matrix) {
        List spirals = new ArrayList<>();
        if (matrix.length == 0 || matrix[0].length == 0) return spirals;
        spiral(matrix, 0, 0, matrix[0].length-1, matrix.length-1, spirals);
        return spirals;
    }
}