题解 | #实现二叉树先序，中序和后序遍历#

using System;
using System.Collections.Generic;

/*
public class TreeNode
{
    public int val;
    public TreeNode left;
    public TreeNode right;

    public TreeNode (int x)
    {
        val = x;
    }
}
*/

class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类 the root of binary tree
     * @return int整型二维数组
     */
    public List<List<int>> threeOrders (TreeNode root) {
        // write code here
        if (root == null)
            return new List<List<int>>() {
            new List<int>(), new List<int>(), new List<int>()
        };
        List<int> listLeft = new List<int>();
        DGLeft(root, listLeft);
        List<int> listMiddle = new List<int>();
        DGMiddle(root, listMiddle);
        List<int> listRight = new List<int>();
        DGRight(root, listRight);
        // write code here
        return new List<List<int>>() {
            listLeft, listMiddle, listRight
        };
    }

    public void DGLeft(TreeNode treeNode, List<int> listInt) {
        if (treeNode == null)
            return;
        listInt.Add(treeNode.val);
        DGLeft(treeNode.left, listInt);
        DGLeft(treeNode.right, listInt);
    }

    public void DGMiddle(TreeNode treeNode, List<int> listInt) {
        if (treeNode == null)
            return;
        DGMiddle(treeNode.left, listInt);
        listInt.Add(treeNode.val);
        DGMiddle(treeNode.right, listInt);
    }

    public void DGRight(TreeNode treeNode, List<int> listInt) {
        if (treeNode == null)
            return;
        DGRight(treeNode.left, listInt);
        DGRight(treeNode.right, listInt);
        listInt.Add(treeNode.val);
    }
}