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
 * 就是写了三道简单题，二叉树前序中序后序遍历
 */
public int[][] threeOrders (TreeNode root) {
    // write code here
    preorder(root, list1);
    inorder(root, list2);
    afterorder(root, list3);
    
    int n = list1.size();
    int[][] arr = new int[3][n];
    for(int i = 0; i < n; i++){
        arr[0][i] = list1.get(i);
        arr[1][i] = list2.get(i);
        arr[2][i] = list3.get(i);
    }
    return arr;
}
List<Integer> list1 = new ArrayList<>();
public void preorder(TreeNode root, List<Integer> list){
    if(root == null) return;
    list1.add(root.val);
    preorder(root.left, list);
    preorder(root.right, list);
}
List<Integer> list2 = new ArrayList<>();
public void inorder(TreeNode root, List<Integer> list){
    if(root == null) return;
    inorder(root.left, list);
    list2.add(root.val);
    inorder(root.right, list);
}
List<Integer> list3 = new ArrayList<>();
public void afterorder(TreeNode root, List<Integer> list){
    if(root == null) return;
    afterorder(root.left, list);
    afterorder(root.right, list);
    list3.add(root.val);
}

}