[LeetCode][JavaScript]Binary Tree Preorder Traversal

Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree

{1,#,2,3}

,

1
\
2
/
3

return

[1,2,3]

.

Note: Recursive solution is trivial, could you do it iteratively?

https://leetcode.com/problems/binary-tree-preorder-traversal/

树的先序遍历，不递归的方式，维护一个栈，先塞右子树，再塞左子树。

Binary Tree Inorder Traversal:

/article/7171024.html

Binary Tree Postorder Traversal：

/article/7171025.html

递归：

/**
* Definition for a binary tree node.
* function TreeNode(val) {
*     this.val = val;
*     this.left = this.right = null;
* }
*/
/**
* @param {TreeNode} root
* @return {number[]}
*/
var preorderTraversal = function(root) {
var res = [];
preorder(root);
return res;

function preorder(node){
if(node && node.val !== undefined){
res.push(node.val);
if(node.left !== null){
preorder(node.left);
}
if(node.right !== null){
preorder(node.right);
}
}
}
};

非递归：

/**
* @param {TreeNode} root
* @return {number[]}
*/
var preorderTraversal = function(root) {
var stack = [], res = [], curr;
if(root){
stack.push(root);
}
while(stack.length !== 0){
curr = stack.pop();
res.push(curr.val);
if(curr.right !== null){
stack.push(curr.right);
}
if(curr.left !== null){
stack.push(curr.left);
}
}
return res;
};