方法一：（递归）
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode() {} * TreeNode(int val) { this.val = val; } * TreeNode(int val, TreeNode left, TreeNode right) { * this.val = val; * this.left = left; * this.right = right; * } * } */ class Solution { public static void pre(TreeNode root,List<Integer> res) { if(root==null) { return; } res.add(root.val); pre(root.left,res); pre(root.right,res); } public List<Integer> preorderTraversal(TreeNode root) { List<Integer> res=new ArrayList<Integer>(); pre(root,res); return res; } }方法二：（迭代）
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode() {} * TreeNode(int val) { this.val = val; } * TreeNode(int val, TreeNode left, TreeNode right) { * this.val = val; * this.left = left; * this.right = right; * } * } */ class Solution { public List<Integer> preorderTraversal(TreeNode root) { List<Integer> res=new ArrayList<Integer>(); LinkedList<TreeNode> queue=new LinkedList<TreeNode>(); if(root==null) { return res; } queue.add(root); while(!queue.isEmpty()) { TreeNode node=queue.pollLast(); res.add(node.val); if(node.right!=null) { queue.add(node.right); } if(node.left!=null) { queue.add(node.left); } } return res; } }