递归和迭代转化思路

整体思路

/**
 * 迭代实现
 */
function demo1 () {
  // 前置代码
  for (初始代码; 循环条件; 迭代代码) {
    // 循环体
  }
  // 后置代码
}
/**
 * 递归实现
 */
function demo2 () {
  // 前置代码
  初始代码;
  _demo2(){
    if (!循环条件) return
    // 循环体
    迭代代码;
    _demo2()
  }
  _demo2()
  // 后置代码
}

1、数组求和

let arr = [1, 2, 3, 4]
function sum1 (arr) {
  let sum = 0
  for (let i = 0; i < arr.length; i++) {
    sum += arr[i]
  }
  return sum
}
console.log(sum1(arr))
// 递归
function sum2 (arr) {
  let sum = 0
  let i = 0
  function sum2 () {
    if (!(i < arr.length)) return
    sum += arr[i]
    i++
    sum2()
  }
  sum2()
  return sum
}
console.log(sum2(arr))

2、数组扁平化

let arr = [1, [1, 2]]
function flatten (arr) {
  let stack = [...arr]
  let res = []
  while (stack.length) {
    const node = stack.pop()
    if (Array.isArray(node)) {
      stack.push(...node)
    } else {
      res = [...res, node]
    }
  }
  return res
}
console.log(flatten(arr))

function flatten2 (arr) {
  let stack = [...arr]
  let res = []
  function _flatten () {
    if (!stack.length) return
    const node = stack.pop()
    if (Array.isArray(node)) {
      stack.push(...node)
    } else {
      res = [...res, node]
    }
    _flatten()
  }
  _flatten()
  return res
}
console.log(flatten2(arr))

3、树转数组

let tree = [
  {
    id: "1",
    pid: "-1",
    children: [
      {
        id: "2",
        pid: "1",
      }
    ]
  }
]
function treeToArray (tree) {
  let res = []
  let stack = [...tree]
  function _treeToArray () {
    if (!stack.length) return
    const node = stack.pop()
    const { children, ...rest } = node
    res.push(rest)
    if (node.children) {
      stack.push(...children)
      _treeToArray()
    }
  }
  _treeToArray()
  return res
}
console.log(treeToArray(tree))
function treeToArray2 (tree) {
  let res = []
  let stack = [...tree]
  while (stack.length) {
    const node = stack.pop()
    const { children, ...rest } = node
    res.push(rest)
    if (node.children) {
      stack.push(...children)
    }
  }
  return res
}
console.log(treeToArray2(tree))

3、数组转树

let arr = [{ id: '1', pid: '-1' }, { id: '2', pid: '1' }]
function arrayToTree (arr) {
  let map = arr.reduce((acc, cur) => {
    acc[cur.id] = cur
    return acc
  }, {})
  arr.forEach(ele => {
    if (map[ele.pid]) {
      map[ele.pid].children = map[ele.pid].children || []
      map[ele.pid].children.push(ele)
      delete map[ele.id]
    }
  })
  return Object.values(map)
}
console.log(JSON.stringify(arrayToTree(arr))) 

function arrayToTree2 (arr) {
  let map = arr.reduce((acc, cur) => {
    acc[cur.id] = cur
    return acc
  }, {})
  function _arrayToTree (i) {
    if (i >= arr.length) return
    const ele = arr[i]
    if (map[ele.pid]) {
      map[ele.pid].children = map[ele.pid].children || []
      map[ele.pid].children.push(ele)
      delete map[ele.id]
    }
    _arrayToTree(++i)
  }
  _arrayToTree(0)
  return Object.values(map)
}
console.log(JSON.stringify(arrayToTree2(arr))) 

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