各种算法（一）

求1+2+3+…+n

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

统计一个数字在无序数组中出现的次数

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

找出数组中任意一个重复的数字

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

快速排序

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));
}