94. Binary Tree Inorder Traversal

//Solution 1：递归

 

public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> list =new ArrayList<>();
    addNode(list, root);
    return list;
}

private void addNode(List<Integer> list, TreeNode node) {
    if (node == null)
        return;
    addNode(list, node.left);
    list.add(node.val);
    addNode(list, node.right);
}

 

//Solution 2：迭代

public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> list =new ArrayList<>();
    if (root == null)
        return list;
    Stack<TreeNode> s = new Stack<>();
    s.push(root);
    while (!s.empty()) {
        TreeNode tn = s.peek();
        while( tn != null) {
            tn = tn.left;
            s.push(tn);
        }
        s.pop();
        if (!s.isEmpty()) {
            tn = s.pop();
            list.add(tn.val);
            s.push(tn.right);
        }
    }
    return list;
}