中序遍历-二叉树（来源LeetCode）

给定一个二叉树，返回它的中序 遍历。

示例:

输入: [1,null,2,3]
1

2
/
3

输出: [1,3,2]

实现代码：

import java.util.*;
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}     //树节点，包含当前节点的值和左右子树（迭代产生），并包含构造方法对val赋值


public class Solution {
    public static List<Integer> inorderTraversal(TreeNode root){
        List<Integer> list = new ArrayList<>();
        preHeader(root,list);
        return list;
    }
    private static void preHeader(TreeNode root ,List<Integer> list){
        if(root == null) return;
        preHeader(root.left,list);
        list.add(root.val);
        preHeader(root.right,list);

    }
    public static void main(String [] args){
        TreeNode a = new TreeNode(1);
        TreeNode b = new TreeNode(3);
        TreeNode c = new TreeNode(2);
        a.right = b;
        b.left = c;
        List<Integer> list1 = new ArrayList<>();
        list1 = inorderTraversal(a);
        System.out.println(list1);

    }
}