https://leetcode-cn.com/problems/shun-shi-zhen-da-yin-ju-zhen-lcof/
func spiralOrder(_ matrix: [[Int]]) -> [Int] {
if matrix.count <= 0 {return []}
var left = 0
var top = 0
var right = matrix[0].count - 1
var bottom = matrix.count - 1
var result = [Int]()
while true {
//left->right
for i in left...right {result.append(matrix[top][i])}
top = top + 1
if top > bottom {break}
//top->bottom
for i in top...bottom {result.append(matrix[i][right])}
right = right - 1
if left > right {break}
//right->left
for i in stride(from: right, to: left - 1, by: -1) {result.append(matrix[bottom][i])}
bottom = bottom - 1
if top > bottom {break}
//bottom->top
for i in stride(from: bottom, to: top - 1, by: -1) {result.append(matrix[i][left])}
left = left + 1
if left > right {break}
}
return result
}
