各种算法（一）

function Sum_Solution(n) {
    var result = n;
    if(n > 0)
        result += Sum_Solution(n-1);
    return result;
}
console.log(Sum_Solution(3)); //6

function count(arr,num){
    arr.sort(function(a,b){
      return a-b;
  });
  var result;
  var left = arr.indexOf(num);
 var right = arr.lastIndexOf(num,arr.length-1);
   if(arr.indexOf(num) == -1){
      result = 0;
  }else{
       result = right-left+1;
   }
    return result;
}
console.log(count([5,5,5,5,5,2,3],5)); //5

function duplicate(numbers, duplication){
     // write code here
    //这里要特别注意~找到任意重复的一个值并赋值到duplication[0]
    //函数返回True/False
    var temp = [];
    for(var i in numbers){
        if(temp.indexOf(numbers[i])==-1){
            temp.push(numbers[i]);
        }else{
            duplication[0] = numbers[i];
            return true;
        }
    }
    return false;
}
console.log(duplicate([5,5,5,5,5,2,3],[]));//true

function quickSort(arr){
    if(arr.length <= 1){
        return arr;
    }
    var pivotIndex = Math.floor(arr.length/2);
    var pivot = splice(pivotIndex,1)[0];
    var left = [];
    var right = [];
    for(var i in arr){
        if(arr[i] < pivot){
            left.push(arr[i]);
        }else{
            right.push(arr[i]);
        }
    }
    return quickSort(left).concat([pivot],quickSort(right));
}