js排列组合

var arr = [1, 2, 3, 4]

function qpl(arr, ans) {
    if (arr.length <= 0) {
        console.log(ans)
        return
    }
    for (var i = 0, len = arr.length; i < len; i++) {
        ans.push(arr[i])
        qpl(arr.slice(0, i).concat(arr.slice(i + 1)), ans)
        ans.pop()
    }
}
// qpl(arr, [])

function qpl2(arr, ans) {
    if (arr.length <= 1) {
        console.log(ans)
        return
    }
    for (var i = 0, len = arr.length; i < len; i++) {
        ans.push(arr[i])
        qpl2(arr.slice(0, i).concat(arr.slice(i + 1)), ans)
        ans.pop()
    }
}
qpl2(arr, [])