二叉树定义

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;
    }
}

前序遍历

public List<Integer> preorder(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    preorderHelper(root, res);
    return res;
}
private void preorderHelper(TreeNode node, List<Integer> res) {
    if (node == null) return;
    res.add(node.val);
    preorderHelper(node.left, res);
    preorderHelper(node.right, res);
}

中序遍历

public List<Integer> inorder(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    inorderHelper(root, res);
    return res;
}
private void inorderHelper(TreeNode node, List<Integer> res) {
    if (node == null) return;
    inorderHelper(node.left, res);
    res.add(node.val);
    inorderHelper(node.right, res);
}

后序遍历

public List<Integer> postorder(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    postorderHelper(root, res);
    return res;
}
private void postorderHelper(TreeNode node, List<Integer> res) {
    if (node == null) return;
    postorderHelper(node.left, res);
    postorderHelper(node.right, res);
    res.add(node.val);
}