二叉树前序、中序、后序遍历（Java递归版）

Binary Tree Preorder Traversal

public class Solution {
    public List preorderTraversal(TreeNode root) {
        List result = new ArrayList();
        helper(root, result);
        return result;
    }
    private static void helper(TreeNode root, List result) {
        if (root == null) {
            return;
        }
        result.add(root.val);
        if (root.left != null) {
            helper(root.left, result);
        }
        if (root.right != null) {
            helper(root.right, result);
        }
    }
}

Binary Tree Inorder Traversal

public class Solution {
    public List inorderTraversal(TreeNode root) {
        List result = new ArrayList();
        helper(root, result);
        return result;
    }
    private static void helper(TreeNode root, List result) {
        if (root == null) {
            return;
        }
        if (root.left != null) {
            helper(root.left, result);
        }
        result.add(root.val);
        if (root.right != null) {
            helper(root.right, result);
        }
    }
}

Binary Tree Postorder Traversal

public class Solution {
    public List postorderTraversal(TreeNode root) {
        List result = new ArrayList();
        helper(root, result);
        return result;
    }
    private static void helper(TreeNode root, List result) {
        if (root == null) {
            return;
        }
        if (root.left != null) {
            helper(root.left, result);
        }
        if (root.right != null) {
            helper(root.right, result);
        }
        result.add(root.val);
    }
}