Leetcode_#144_二叉树的前序遍历

原题：#144_二叉树的前序遍历

public List<Integer> f (TreeNode root) {
    List<Integer> ret = new ArrayList<>();
    Stack<TreeNode> stack = new Stack<>();
    if (root == null) return ret;
    stack.push(root);
    while (!stack.isEmpty()) {
        TreeNode node = stack.pop();
        ret.add(node.val);
        if (node.right != null) stack.push(root.right);
        if (node.left != null) stack.push(root.left);
    }
    return ret;
}