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 class Solution {
    /**
     * 
     * @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
        List<Integer> p1 = new ArrayList();
        preOrders(root, p1);

        List<Integer> p2 = new ArrayList();
        middOrders(root, p2);

        List<Integer> p3 = new ArrayList();
        postOrders(root, p3);

        int[][] result = new int[3][p1.size()];
        for(int i=0; i<p1.size(); i++) {
            result[0][i] = p1.get(i);
            result[1][i] = p2.get(i);
            result[2][i] = p3.get(i);
        }
        return result;
    }

    private void preOrders (TreeNode root, List<Integer> result) {
        if (root!=null) {
            result.add(root.val);
            preOrders(root.left, result);
            preOrders(root.right, result);
        } 
    }

     private void middOrders (TreeNode root, List<Integer> result) {
        if (root!=null) {
            middOrders(root.left, result);
            result.add(root.val);
            middOrders(root.right, result);
        } 
    }

    private void postOrders(TreeNode root, List<Integer> result) {
        if (root!=null) {
            postOrders(root.left, result);
            postOrders(root.right, result);
            result.add(root.val);
        } 
    }
}