leetcode [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?

题目大意：

使用非递归的解法，得到二叉树的前序遍历。

解法：

使用栈来做

java:

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer>res=new ArrayList<Integer>();
        Stack<TreeNode>rights=new Stack<>();
        while(root!=null){
            res.add(root.val);
            if(root.right!=null) rights.add(root.right);
            root=root.left;
            if(root==null&&!rights.empty()) root=rights.pop();
        }

        return res;
    }
}