var arr = [4, 1, 6, 9, 3, 2, 8, 7];

    function quickSort(arr) {
    if (arr == null || arr.length == 0) return [];
    //选班长
    var leader = arr[0];
    //小的站左边，大的站右边
    var left = [];
    var right = [];
    for (var i = 1; i < arr.length; i++) {
    if (arr[i] < leader) left.push(arr[i]);
    else right.push(arr[i]);
    }
    left = quickSort(left);
    right = quickSort(right);
    left.push(leader);
    return left.concat(right);
    }
    // quickSort(arr);
    console.log(quickSort(arr));