代码

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

//中序
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        travel(root, list);
        return list;
    }
    public void travel(TreeNode root, List<Integer> list) {
        if (root == null) 
            return;
        travel(root.left, list);
        list.add(root.val);
        travel(root.right, list);
    }
}

//后序
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        travel(root, list);
        return list;
    }
    public void travel(TreeNode root, List<Integer> list) {
        if (root == null) 
            return;
        travel(root.left, list);
        travel(root.right, list);
        list.add(root.val);
    }
}