二叉树的前序遍历 #2

var preorderTraversal = function(root) {
   let res = []
   function fn (node) {
       if(node) {
           res.push(node.val)
       }
       if (node.left) {
           fn(node.left)
       }
       if (node.right) {
           fn(node.right)
       }
   }
   root && fn(root)
   return res
};

var preorderTraversal = function(root) {
   let res = []
   let arr = []
   if(root) {
       arr.push(root)
   }
   while(arr.length) {
       let n = arr.pop()
       res.push(n.val)
       if (n.right) {
           arr.push(n.right)
       }
       if (n.left) {
           arr.push(n.left)
       }
   }
   return res
};