题解 | #实现二叉树先序，中序和后序遍历#

JAVA

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
     */
    List<Integer> front = new ArrayList<>();
    List<Integer> mid = new ArrayList<>();
    List<Integer> back = new ArrayList<>();

    public int[][] threeOrders (TreeNode root) {
        // write code here
        frontOrder(root);
        midOrder(root);
        backOrder(root);
        //申明二维数组、
        int[][] res = {convertListToArr(front),convertListToArr(mid),convertListToArr(back)};
        return res;
    }

    //先序遍历
    public void frontOrder (TreeNode root){
        if(root ==null) return;
        front.add(root.val);
        frontOrder(root.left);
        frontOrder(root.right);


    }

    //中序遍历
    public void midOrder (TreeNode root){
        if(root ==null) return;
        midOrder(root.left);
        mid.add(root.val);
        midOrder(root.right);
    }

    //后序遍历
    public void backOrder (TreeNode root){
        if(root ==null) return;
        backOrder(root.left);
        backOrder(root.right);
        back.add(root.val);
    }

    //别的转换的接口咱不知道，老老实实写一个list转数组的方法
    public int[] convertListToArr (List<Integer> list){
        int[] arr = new int[list.size()];
        for(int i=0;i<list.size();i++){
            arr[i] = list.get(i);
        }
        return arr;
    }
}