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 */

 //用遍历的方法实现先序，中序和后序遍历

static ArrayList<Integer> list1 = new ArrayList<Integer>();
static ArrayList<Integer> list2 = new ArrayList<Integer>();
static ArrayList<Integer> list3 = new ArrayList<Integer>();

public int[][] threeOrders (TreeNode root) {
    // write code here
    preOrder(root);
    inOrder(root);
    posOrder(root);
    int[][] result = new int[3][list1.size()];
    
    for(int j = 0; j < list1.size(); j++){
        result[0][j] = list1.get(j);
        result[1][j] = list2.get(j);
        result[2][j] = list3.get(j);
    }
    
    return result;
}


public static void preOrder (TreeNode root){
    if(root == null){
        return;
    }
    list1.add(root.val);
    preOrder(root.left);
    preOrder(root.right);
}
public static void inOrder (TreeNode root){
    if(root == null){
        return;
    }
    inOrder(root.left);
    list2.add(root.val);
    inOrder(root.right);
}
public static void posOrder (TreeNode root){
    if(root == null){
        return;
    }
    posOrder(root.left);
    posOrder(root.right);
    list3.add(root.val);
}

}