LeetCode-94. 二叉树的中序遍历

94. 二叉树的中序遍历

给定一个二叉树，返回它的中序 遍历。

示例:

image.png

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

    public static void main(String[] args) {
        S01_binary_tree_inorder_traversal sol = new S01_binary_tree_inorder_traversal();
        TreeNode node1 = sol.new TreeNode(1);
        TreeNode node2 = sol.new TreeNode(2);
        TreeNode node3 = sol.new TreeNode(3);
        node1.left = null;
        node1.right = node2;
        node2.left = node3;
        node2.right = null;
        List list = sol.inorderTraversal(node1);
        System.out.println(list.toString());
    }

    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) {
            val = x;
        }
    }
    public List<Integer> inorderTraversal(TreeNode root) {
        List list = new ArrayList();
        helper(root,list);
        return list;
    }

    void helper(TreeNode root,List list){
        if(root!= null){
            if(root.left != null){
                helper(root.left,list);
            }
            list.add(root.val);
            if(root.right != null){
                helper(root.right,list);
            }
        }

    }