function quickSort(arr){
if(arr.length === 0) return [];
var l = [];
var r= [];
var m = ~~(arr.length/2);
var flag = arr.splice(m,1);
for(let i = 0; i < arr.length; i++){
arr[i] < flag ? l.push(arr[i]): r.push(arr[i]);
}
return quickSort(l).concat(flag,quickSort(r));
}
quickSort([5,4,3,2,7,1]) // [1,2,3,4,5,7]
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