简单递归实现：

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

    public void postorderTraversal(List<Integer> result, TreeNode root){
        if(root==null)
            return;
        postorderTraversal(result, root.left);
        postorderTraversal(result, root.right);
        result.add(root.val);
    }

    public void inorderTraversal(List<Integer> result, TreeNode root){
        if(root==null)
            return;
        inorderTraversal(result, root.left);       

        result.add(root.val);
        inorderTraversal(result, root.right);   
    }

     
}

数据结构复习--binary tree pre-order / post-order / in-order traversal