1. 計算1到N總和(1 + 2 + 3 +...+N)

遞迴解法

function sum(n){
  if(n<1){
    return n
  }
  return n + sum(n-1);
}

var total = sum(10);
console.log("total:" + total);

迴圈版本

function sumByFor(n){
   var total = 0;
  for(i=1;i<=n;i++){
    total+=i
  }
  return total;
}
var total = sumByFor(10);
console.log("total by for loop:" + total);

2. 1*2+2*3+...+(n-1)*n

遞迴解法

function sumContinue(n){
  if(n<2){
    return 0
  }
  return n*(n-1) + sumContinue(n-1);
}

var total = sumContinue(10);
console.log("total continue:" + total);

迴圈解法

function sumContinueByFor(n){
  var total =0;
  for(i=2;i<=n;i++){
    total +=i*(i-1);
  }
  return total;
}
var total = sumContinueByFor(10);
console.log("sumContinue by for loop:" + total);

3.不使用暫存變數交換兩個變數 ( Swap two variables without using a temporary variable )

XOR交換方式

a =3;
b = 4;
a = a ^ b;
b = a ^ b;
a = a ^ b;
console.log("a:"+ a + " b:"+b);

4.從陣列中找出缺少的數字

const findMissing = (data) => {
  let max = data[0]
  let min = data[0]
  let sum = 0;
  for (i = 0; i < data.length; i++) {
    let n = data[i];
    min = (data[i]>min) ? min :data[i]
    max = (data[i]>max) ? data[i]: max 
    sum+= n;
  }
  return (max+min)* (max - min + 1) /2 - sum;
};

let value = findMissing([3,4,6,7,8,9])
console.log("missing value: "+ value)

5.單迴圈做出99乘法表

let i = 9;
let j = i;
while(i>0){
  console.log(i+"*"+j + "="+ i*j)
  j--;
  if(j<1){
    i--;
    j=9;
  }
}