递归

数字求和

    function sum(n){

      if(n===1){

        return n=1

      }

      return n+sum(n-1)  

}

每次执行

5+sum(n-1)  //n=5

5+4+sum(n-1) //n=4

5+4+3+sum(n-1)  //n=3

5+4+3+2+sum(n-1) //n=2

5+4+3+2+1+sum(n-1) //n=1

5+4+3+2+1

console.log(sum(5)); //15

---

数组求和

     function sum(arr) {

        var len = arr.length

        if (len == 0) {

          return 0

        } else if (len == 1) {

          return arr[0]

        } else {

          return arr[0] + sum(arr.slice(1))  //每一次调用就把数组的第一项去除再次传入进去

        }

      }

      let arr = [ 1, 2, 3, 4, 5 ]  

每次执行

1+sum(2,3,4,5)

1+2+sum(3,4,5)

1+2+3+sum(4,5)

1+2+3+4+sum(5)

1+2+3+4+5

1+2+3+9

1+2+12

1+14

      console.log(sum(arr))

---

数组转树

        let data = [{

                id: "01",

                pid: "",

"name": "老王"

            },

            {

                id: "02",

                pid: "01",

"name": "小张"

            }

---

        ]

  function fn(data, rootId = '') {

const children = []      //定义一个空数组

            data.forEach(item => {

if (item.pid === rootId) {    //如果每一项的pid=rootId就添加到数组里

                    children.push(item)

                    item.children = fn(data, item.id)

                }

            });

            return children

        }