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
        //三种储存方式列表
        List<Integer> frontList = new ArrayList<Integer>();
        List<Integer> centreList = new ArrayList<Integer>();
        List<Integer> rearList = new ArrayList<Integer>();
        //前序方式遍历：先根后左最右
        frontTraversal(root,frontList);
        //中序方式遍历：先左后根最右
        centreLTraversal(root,centreList);
        //后序方式遍历：先左后右最根
        rearTraversal(root,rearList);

        //储存结果集到二维数组中
        int [][] roots = new int[3][frontList.size()];
        for(int i = 0; i < frontList.size(); i++){
            roots[0][i] = frontList.get(i);
            roots[1][i] = centreList.get(i);
            roots[2][i] = rearList.get(i);
        }
        return roots;
    }
    private void frontTraversal(TreeNode root,List<Integer> list){
        if(root == null){
            return;
        }
        list.add(root.val);
        frontTraversal(root.left,list);
        frontTraversal(root.right,list);
    }
    private void centreLTraversal(TreeNode root,List<Integer> list){
        if(root == null){
            return;
        }
        centreLTraversal(root.left,list);
        list.add(root.val);
        centreLTraversal(root.right,list);
    }
    private void rearTraversal(TreeNode root,List<Integer> list){
        if(root == null){
            return;
        }
        rearTraversal(root.left,list);
        rearTraversal(root.right,list);
        list.add(root.val);
    }
}