数组扁平化

 let arr = [1, [2, [1, 2, 3, [4, 5, 6]]], 18, [25, 12, [13, 14, 27]]]
        function flatten(arr) {
            let res = []
            for (let i = 0; i < arr.length; i++) {
                if (Array.isArray(arr[i])) {
                    res = res.concat(flatten(arr[i]))
                } else {
                    res.push(arr[i])
                }
            }
            return res
        }
        console.log(flatten(arr));

let arr = [1, [2, [1, 2, 3, [4, 5, 6]]], 18, [25, 12, [13, 14, 27]]]
        function flatten(arr, k) {
            if (k == 0) {
                return arr
            }
            let res = []
            for (let i = 0; i < arr.length; i++) {
                if (Array.isArray(arr[i])) {
                    res = res.concat(flatten(arr[i], k - 1))
                } else {
                    res.push(arr[i])
                }
            }
            return res
        }
        console.log(flatten(arr, 2));