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
        int[][] a = new int[3][3];
        for (int i = 0; i < 3; i++) {
            int[] b = {};
            if (root == null) {
                a[i] = b;
                continue;
            }
            List<Integer> list = new ArrayList();
            if (i == 0) {
                list = xian(root);
            } else if (i == 1) {
                list = zhon(root);
            } else {
                list = hou(root);
            }
            b = list.stream().mapToInt(Integer::valueOf).toArray();
            a[i]=b;
        }
        return a;
    }
    public List<Integer> xian(TreeNode root) {
        List<Integer> a = new ArrayList();
        if (root != null) {
            a.add(root.val);
            a.addAll(xian(root.left));
            a.addAll(xian(root.right));
        }
        return a;
    }
    public List<Integer> zhon(TreeNode root) {
        List<Integer> a = new ArrayList();
        if (root != null) {
            a.addAll(zhon(root.left));
            a.add(root.val);
            a.addAll(zhon(root.right));
        }
        return a;
    }
    public List<Integer> hou(TreeNode root) {
        List<Integer> a = new ArrayList();
        if (root != null) {
            a.addAll(hou(root.left));
            a.addAll(hou(root.right));
            a.add(root.val);
        }
        return a;
    }
}

先序就是中左右，

中序就是左中右，

后序就是左右中，

记死就行。