一、前序遍历

const preorderTraversal = (root) => {
    const list = []
  const stack = []
  if(root) {
      stack.push(root)
  }
  while(stack.length > 0) {
      let curNode = stack.pop()
    list.push(curNode.val)
    if(curNode.right !== null) {
        stack.push(curNode.right)
    }
    if(curNode.left !== null) {
        stack.push(curNode.left)
    }
  }
  return list
}

二、中序遍历

const inorderTraversal = (root) => {
    const list = []
  const stack = []
  let node = root
  while(node || stack.length){
      while(node) {
        stack.push(node)
      node = node.left
    }
    node = stack.pop()
        list.push(node.val)
    node = node.right
  }
  reutrn list
}

三、后序遍历

const postorderTraversal = (root) => {
    const list = []
  const stack = []
  if(root) {
      stack.push(root)
  }
  while(stack.length > 0) {
      let curNode = stack.pop()
    list.unshift(curNode.val)
     if(curNode.left !== null) {
        stack.push(curNode.left)
    }
    if(curNode.right !== null) {
        stack.push(curNode.right)
    } 
  }
  return list
}

四、二叉树的深度

const maxDepth = function(root) {
    if(root == null) {
        return 0
    }else{
        let left = maxDepth(root.left)
        let right = maxDepth(root.right)
        return Math.max(left, right) + 1
    }
};

五、二叉树层序遍历

//BFS
var levelOrder = function(root) {
   if(root == null) {
        return []
    }
    let res = []
    let queue = [root]
    while(queue.length > 0) {
        let len = queue.length
        let arr = []
        while(len) {
            let node = queue.shift()
            arr.push(node.val)
            if(node.left) {
                queue.push(node.left)
            }
            if(node.right) {
                queue.push(node.right)
            }
            len--
        }
        res.push(arr)
    }
    return res
}

六、翻转二叉树

var invertTree = function(root) {
    if(root == null) {
        return null
    }
    let left = invertTree(root.left)
    let right = invertTree(root.right)
    root.left = right
    root.right = left
    return root
};