https://blog.youkuaiyun.com/weixin_45334589/article/details/136220521?spm=1001.2014.3001.5502
通过上文构建二叉树
//前序遍历 中->左->右
var preorderTraversal = function(root){
const stack = [root]; //头节点进栈
var res = []; //保存结果
while(stack.length>0){
var cur = stack.pop(); //保存栈顶元素
res.push(cur.val) //将栈顶元素保存结果集
//因为栈的性质先进后出,所以先右到左入栈,出栈时就是左到右
if(cur.right!=null){ //如果栈顶节点存在左右节点,入栈
stack.push(cur.right)
}
if(cur.left!=null){
stack.push(cur.left)
}
}
return res
}