export default (matrix) => { 
    if(matrix.length === 0) return []
    let result = []
    // 处理每一圈的数据遍历过程
    let map = (arr) => {
        if(arr.length === 1) {
            result = result.concat(arr.shift())
            return
        } 
        if(arr.length === 2) {
            result = result.concat(arr.shift())
            result = result.concat(arr.pop().reverse())
            return
        }
        if(arr.length > 2) {
            result = result.concat(arr.shift())
            for (let i = 0, n = arr.length; i < n; i++) {
                result.push(arr[i].pop())
            }
            result = result.concat(arr.pop().reverse())
            for(let i = arr.length - 1; i >= 0; i--) {
                result.push(arr[i].shift())
            }
            map(arr)    
        }
    }
    map(matrix)
    result = result.filter(item => item !== undefined)
    return result
};