Leetcode二叉树算法个人理解二叉树的前中后序遍历递归写法 树节点 class TreeNode { int val; TreeNode left; TreeNode right; TreeNode(int val) { this.val = val; } } 前序 class Solution { public List preorderTraversal(TreeNode root) { List res = new ArrayList<>(); dfs(root, res); return res; } private void