先序遍历： 根节点-》左子树-》右子树

    1. const preorder = (root) => {
    2.     if(!root) return
    3.   console.log(root.val)
    4.   preorder(root.left)
    5.   preorder(root.right)
    6. }

    先序遍历非递归版

    1. const preorder = (root) => {
    2.     if(!root) return
    3.   const stack = [root]
    4.   while(stack.length) {
    5.       const n = stack.pop()
    6.     console.log(n.value)
    7.     if(n.right) stack.push(n.right)
    8.     if(n.left) stack.push(n.left)
    9.   }
    10. }

    中序遍历 左子树-》根结点-》右子树

    1. function inorder (root) {
    2.   if(!root) return;
    3.     preorder(root.left)
    4.   console.log(root.val)
    5.   preorder(root.right)
    6. }

    非递归版本

    1. function inorder(root) {
    2.     if(!root) return;
    3.   const stack = []
    4.   const p = root
    5.   while(stack.length || p) {
    6.       while(p) {
    7.         stack.push(p)
    8.       p = p.left
    9.     }
    10.     const n = stack.pop()
    11.     console.log(n.val)
    12.     p = p.right
    13.   }
    14. }

    后序遍历： 右子树 =》 根节点 =》 左子树

    1. const postOrder = （root) => {
    2.     if(!root) return;
    3.   postOrder(root.right)
    4.   console.log(root.value)
    5.   preOrder(root.left)
    6. }

    后序遍历非递归

    1. const postOrder = (root) => {
    2.     if(!root) return
    3.   const stack = [root]
    4.   const outputStack = []
    5.   while(stack.lenght) {
    6.       const n = stack.pop()
    7.     outputStack.push(n)
    8.     if(n.left) stack.push(n.left)
    9.     if(n.right) stack.push(n.right)
    10.   }
    11.   while(outputStack.length) {
    12.     const n = outputStack.pop()
    13.     console.log(n)
    14.   }
    15. }