94. 二叉树的中序遍历

​​https://leetcode-cn.com/problems/binary-tree-inorder-traversal/​​

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
 //中序，左右根
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        inorder(root,res);
        return res;
    }
    //确认递归函数的参数和返回值
    public void inorder(TreeNode root,List<Integer> res){
        //确定终止条件
        if(root == null){
            return;
        }
        //确认单层递归的逻辑
        inorder(root.left,res);
        res.add(root.val);
        inorder(root.right,res);
    }
}