题解 | #实现二叉树先序，中序和后序遍历#

其实前序中序后序不过是代码顺序变化而已

import java.util.*;


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) {
        ArrayList array = new ArrayList();
        per(array,root);
        mid(array,root);
        bac(array,root);
        int[][] a = new int[3][array.size()/3];
        // write code here
        int j = 0;
        int k = 0;

        for(int i = 0; i<array.size(); i++){
            a[j][k] = (Integer)array.get(i);

            if(k>=array.size()/3-1){
                j++;
                k = 0;
            }else{
                k++;
            }

        }

        return a ;
    }

    public void per(ArrayList arry,TreeNode node){
        if(node==null){
            return ;
        }
        arry.add(node.val);
        per(arry,node.left);
        per(arry,node.right);

    }
    public void mid(ArrayList arry,TreeNode node){
        if(node==null){
            return ;
        }
        mid(arry,node.left);
        arry.add(node.val);
        mid(arry,node.right);

    }
    public void bac(ArrayList arry,TreeNode node){
        if(node==null){
            return ;
        }
        bac(arry,node.left);
        bac(arry,node.right);
        arry.add(node.val);

    }
}