代码随想录算法训练营第十四天| 二叉树相关

二叉树的递归遍历
递归三要素：确定递归函数的参数和返回值，确定终止条件，确定单层递归的逻辑

144. 二叉树的前序遍历
https://leetcode.cn/problems/binary-tree-preorder-traversal/description/

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