先说二叉树的遍历，遍历方式：

• 前序遍历：先遍历根结点，然后左子树，再右子树
• 中序遍历：先遍历左子树，然后根结点，再右子树
• 后续遍历：先遍历左子树，然后右子树，再根结点

递归

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function(root) {
    const nodeList = []
    const proOrderTraverse = (node)=>{
        if(!node) return 
        nodeList.push(node.val)
        proOrderTraverse(node.left)
        proOrderTraverse(node.right)
    }
    proOrderTraverse(root)
    return nodeList
};

迭代

var preorderTraversal = function(root) {
    const nodeList = []
    const nodeStack = []
    let node = root
    while(nodeStack.length>0 || node){
        while(node){
            nodeList.push(node.val)
            nodeStack.push(node)
            node = node.left
        }
        node = nodeStack.pop()
        node = node.right
    }
    return nodeList
};

morris

var preorderTraversal = function(root) {
    const nodeList = []
    let p1 = root, p2 = null
    while(p1 != null){
        p2 = p1.left
        if(p2 != null){
            while(p2.right != null && p2.right !=p1){
                p2 = p2.right
            }
            if(p2.right == null){
                nodeList.push(p1.val)
                p2.right = p1
                p1 = p1.left
                continue
            } else{
                p2.right = null
            }
        }else {
            nodeList.push(p1.val)
        }
        p1 = p1.right
    }
    return nodeList
};