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
     */

    public int[][] threeOrders (TreeNode root) {
        // write code here
        if(root == null) return new int[0][0];
        List<List<Integer>> l = new ArrayList<>();

        Stack<TreeNode> s = new Stack<>();
        List<Integer> preArr = new ArrayList<Integer>();
        s.push(root);
        while(!s.isEmpty()){
            TreeNode temp = s.pop();
            preArr.add(temp.val);
            if(temp.right != null) s.push(temp.right);
            if(temp.left != null) s.push(temp.left);
        }
        l.add(preArr);
        List<Integer> inArr = new ArrayList<Integer>();
        TreeNode tem = root;
        while(!s.isEmpty() || tem != null){
            if(tem != null) {
                s.push(tem);
                tem = tem.left;
            }
            else {
                tem = s.pop();
                inArr.add(tem.val);
                tem = tem.right;
            }
        }
        l.add(inArr);
        Stack<TreeNode> assistant = new Stack<>();
        s.push(root);
        while(!s.isEmpty()){
            TreeNode temp = s.pop();
            assistant.push(temp);
            if(temp.left != null) s.push(temp.left);
            if(temp.right != null) s.push(temp.right);
        }
        List<Integer> posArr = new ArrayList<Integer>();
        while(!assistant.isEmpty()){
            posArr.add(assistant.pop().val);
        }
        l.add(posArr);
        int len = posArr.size();
        int[][] res = new int[l.size()][len];
        for(int i = 0; i < l.size(); i++){
            for(int j = 0; j < len; j++){
                res[i][j] = l.get(i).get(j);
            }
        }
        return res;
    }
}