Binary Tree Preorder Traversal



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?

解题思路：根左右的形式前序遍历，遍历的元素加入list，左分支遍历结束，继续右分支。

java实现：

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

原题地址：https://leetcode.com/problems/binary-tree-preorder-traversal/

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