leetcode 144. Binary Tree Preorder Traversal ----- java

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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?

 

 

求前序走訪，要求不用遞迴。  使用雙向隊列。 

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List<Integer> preorderTraversal(TreeNode root) {                Deque<TreeNode> queue = new ArrayDeque<TreeNode>();        List list = new ArrayList<Integer>();        if( root == null )            return list;        queue.add(root);        while( !queue.isEmpty() ){            TreeNode node = queue.poll();            list.add(node.val);            if( node.right != null )                queue.addFirst(node.right);            if( node.left != null )                queue.addFirst(node.left);        }        return list;            }}

 

leetcode 144. Binary Tree Preorder Traversal ----- java