leetcode_144_Binary Tree Preorder Traversal

描述：

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?

思路：

代码：

/*** Definition for binary tree* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode(int x) { val = x; }* }*/
public class Solution {public List preorderTraversal(TreeNode root) {Listlist=new ArrayList();if(root==null)return list;Stackst=new Stack();st.push(root);TreeNode top=null;list.add(root.val);while(!st.empty()){top=st.peek();while(top.left!=null){st.push(top.left);list.add(top.left.val);top=top.left;}while(top.right==null){st.pop();if(!st.empty())top=st.peek();elsebreak;}if(!st.empty()){st.pop();st.push(top.right);list.add(top.right.val);}}return list;}
}

结果：

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