二叉树的后续遍历

递归

public List<Integer> preorderTraversal(TreeNode root) {
        if(root == null) return new ArrayList<Integer>();
        
        List<Integer> list = new ArrayList<Integer>();
        preorderTraversal(root, list);
        return list;
    }
    
    public List<Integer> preorderTraversal(TreeNode root, List<Integer> list) {
        
       
        if(root.left != null)
            preorderTraversal(root.left, list);
         list.add(root.val);
        if(root.right != null)
            preorderTraversal(root.right, list);
        return list;
    }

迭代

public List<Integer> postorderTraversal(TreeNode root) {
    Stack<TreeNode> stack = new Stack<>();
    LinkedList<Integer> output = new LinkedList<>();
    if (root == null) {
      return output;
    }

    stack.push(root);
    while (!stack.isEmpty()) {
      TreeNode node = stack.pop();
      output.addFirst(node.val);
      if (node.left != null) {
        stack.push(node.left);
      }
      if (node.right != null) {
        stack.push(node.right);
      }
    }
    return output;
  }