144. Binary Tree Preorder Traversal（js）

144. Binary Tree Preorder Traversal

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

Example:

Input: [1,null,2,3]
   1
    
     2
    /
   3

Output: [1,2,3]

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

题意：二叉树的先序遍历

代码如下：

//法一
// var preorderTraversal = function(root) {
//   let res=[];
//     helper(root,res);
//     return res;
// };
// var helper=function(root,res){
//     if(!root) return ;
//     res.push(root.val);
//     helper(root.left,res);
    
//     helper(root.right,res);
// }
//法二
var preorderTraversal = function(root) {
    if(!root) return [];
    let res=[],s=[];
    s.push(root);
    while(s.length>0){
        let t=s.pop();
        res.push(t.val);
        if(t.right) s.push(t.right);
        if(t.left) s.push(t.left);
    }
    return res;
    
};