题解 | #实现二叉树先序，中序和后序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public 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 int[][] threeOrders (TreeNode root) {
        // write code here
        ArrayList<Integer> in = new ArrayList<>();
        inOrders(root, in);
        ArrayList<Integer> mid = new ArrayList<>();
        midOrders(root, mid);
        ArrayList<Integer> pre = new ArrayList<>();
        preOrders(root, pre);
        int[][] arr = new int[3][in.size()];
        for (int i = 0; i < arr[0].length; i ++) {
            arr[0][i] = in.get(i);
            arr[1][i] = mid.get(i);
            arr[2][i] = pre.get(i);
        }
        return arr;

    }

    // 先序遍历
    public void inOrders(TreeNode root, ArrayList<Integer> list) {
        if (root == null) {
            return;
        }
        list.add(root.val);
        inOrders(root.left, list);
        inOrders(root.right, list);
    }
    // 中序遍历
    public void midOrders(TreeNode root, ArrayList<Integer> list) {
        if (root == null) {
            return;
        }
        midOrders(root.left, list);
        list.add(root.val);
        midOrders(root.right, list);
    }
    // 后序遍历
    public void preOrders(TreeNode root, ArrayList<Integer> list) {
        if (root == null) {
            return;
        }
        preOrders(root.left, list);
        preOrders(root.right, list);
        list.add(root.val);
    }
}