【JAVA】二叉树的非递归前序遍历

	public List preorderTraversal(TreeNode root) {
		System.out.println("前序遍历：");
		List list = new ArrayList<>();
		if (root != null) {
			//利用栈结构的特性，模仿递归遍历
			Stack stack = new Stack();
			//先放入头节点
			stack.push(root);
			//循环的终止条件是栈非空
			while (!stack.isEmpty()) {
				
				TreeNode cur = stack.pop();
				list.add(cur.val);
				if (cur.right != null) {
					stack.push(cur.right);
				}
				if (cur.left != null) {
					stack.push(cur.left);
				}
			}
		}
		return list;
	}