用top，bottom，left，right分别指向上下左右，然后向内循环收缩
    top所在行遍历完后，top+1
    rihgt所在列遍历完后，right-1
    bottom所在行遍历完后，bottom-1
    left所在列遍历完后，left+1

    1. package main
    3. import "fmt"
    4. func spiralOrder(matrix [][]int) []int {
    5.     if len(matrix) == 0 {
    6.         return nil
    7.     }
    8.     left :=0
    9.     right :=len(matrix[0])-1
    10.     top :=0
    11.     bottom :=len(matrix)-1
    12.     grid := make([]int,(right+1)*(bottom+1))
    13.     index :=0
    14.     for left<=right&&top<=bottom{
    15.         // 上层 边界判断
    16.         for i:=left;i<=right&&top<=bottom;i++{
    17.             grid[index] = matrix[top][i]
    18.             index++
    19.         }
    20.         //fmt.Println("top","=",grid)
    21.         top++
    22.         // 右侧
    23.         for i:=top;i<=bottom&&left<=right;i++{
    24.             grid[index]= matrix[i][right]
    25.             index++
    26.         }
    27.         //fmt.Println("right","=",grid)
    28.         right--
    30.         //下
    31.         for i:=right;i>=left&&top<=bottom;i--{
    32.             grid[index]= matrix[bottom][i]
    33.             index++
    34.         }
    35.         //fmt.Println("bottom","=",grid)
    36.         bottom--
    37.         //左
    38.         for i:=bottom;i>=top&&left<=right;i--{
    39.             grid[index]= matrix[i][left]
    40.             index++
    41.         }
    42.         //fmt.Println("left","=",grid)
    43.         left++
    44.         //fmt.Println(grid)
    45.     }
    46.     return grid
    47. }
    50. func main() {
    51.     matrix :=[][]int{{1,2,3,4},{5,6,7,8},{9,10,11,12}}
    52.     fmt.Println("1","=",spiralOrder(matrix))
    54. }

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