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输入：matrix = [[1,2,3],[4,5,6],[7,8,9]]
输出：[1,2,3,6,9,8,7,4,5]

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输入：matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]
输出：[1,2,3,4,8,12,11,10,9,5,6,7]

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m == matrix.length
n == matrix[i].length
1 <= m, n <= 10
-100 <= matrix[i][j] <= 100

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func spiralOrder(matrix [][]int) []int {
	var res []int
	for len(matrix) > 0 {
		var i, j int
		move := right
		for move != 0 {
			res = append(res, matrix[i][j])
			move = next(matrix, i, j, move)
			switch move {
			case right:
				j++
			case down:
				i++
			case left:
				j--
			case up:
				i--
			}
		}
		matrix = rebuild(matrix)
	}
	return res
}

const (
	right = 1
	down  = 2
	left  = 3
	up    = 4
)

// 判断下一步往哪儿走
func next(matrix [][]int, i, j int, move int) int {
	m := len(matrix)
	if m == 0 {
		return 0
	}
	n := len(matrix[0])
	switch move {
	case right:
		if j == n-1 {
			// 如果下一步无法往下走，终止
			if i == m-1 {
				return 0
			}
			return down
		}
	case down:
		if i == m-1 {
			// 如果下一步无法往左走，终止
			if j == 0 {
				return 0
			}
			return left
		}
	case left:
		if j == 0 {
			// 如果下一步无法往上走，终止。这里注意，是1不是0，因为0是起点
			if i == 1 {
				return 0
			}
			return up
		}
	case up: // 走完上就结束了
		if i == 1 {
			return 0
		}
	}
	return move
}

// 矩阵去壳
func rebuild(matrix [][]int) [][]int {
	if len(matrix) == 0 {
		return nil
	}
	if len(matrix) <= 2 || len(matrix[0]) <= 2 {
		return nil
	}
	var res [][]int
	for i := 1; i < len(matrix)-1; i++ {
		res = append(res, matrix[i][1:len(matrix[i])-1])
	}
	return res
}