LeetCode 144 二叉树的前序遍历

给定一个二叉树，返回它的 前序 遍历。

 示例:

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [1,2,3]

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
     class Pair<K,V>{
        K k;
        V v;
        Pair(K k,V v){
            this.k = k;
            this.v = v;
        }

    }
    public List<Integer> preorderTraversal(TreeNode root) {
        if(root==null) return new LinkedList<>();
        Stack<Pair<String,TreeNode>> stack = new Stack();
        stack.push(new Pair("go",root));
        List<Integer> list = new LinkedList<>();
        while (!stack.isEmpty()) {
            Pair<String,TreeNode> treeNode = stack.pop();
            if(treeNode.k.equals("print")){
                list.add(treeNode.v.val);
            }else{
                if(treeNode.v.right!=null){
                    stack.push(new Pair<>("go",treeNode.v.right));
                }
                if(treeNode.v.left!=null){
                    stack.push(new Pair<>("go",treeNode.v.left));
                }
                stack.push(new Pair<>("print",treeNode.v));
            }
        }
        return list;
    }
}