leetcode-树练习-binary-tree-preorder-traversal
binary-tree-preorder-traversal
https://www.nowcoder.com/practice/501fb3ca49bb4474bf5fa87274e884b4?tpId=46&tqId=29036&tPage=1&rp=1&ru=/ta/leetcode&qru=/ta/leetcode/question-ranking
求给定的二叉树的前序遍历。
例如：
给定的二叉树为{1,#,2,3},
非递归前序遍历二叉树
思路和后续是一样的，用两个Stack来辅助我们进行。
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
import java.util.ArrayList;
import java.util.Stack;
public class Solution {
public ArrayList<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> result= new ArrayList<Integer>();
if(root == null)return result;
result.add(root.val);
Stack<TreeNode> stackNode = new Stack<>();
Stack<Integer> stackFlag = new Stack<>();
stackNode.push(root);
stackFlag.push(0);
while(!stackNode.isEmpty()){
TreeNode t
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