实现二叉树前序、中序、后序遍历javascript:void(0);

1. 递归实现前序、中序、后序遍历，用List进行存储
2. Stream流化将List转为int[]

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
     */
    private List<Integer> preOrder = new LinkedList<>();
    private List<Integer> inOrder = new LinkedList<>();
    private List<Integer> postOrder = new LinkedList<>();

    public int[][] threeOrders (TreeNode root) {
        // write code here
        preOrder(root);
        inOrder(root);
        postOrder(root);
        int[] preOrderArray = preOrder.stream().mapToInt(Integer::intValue).toArray();
        int[] inOrderArray = inOrder.stream().mapToInt(Integer::intValue).toArray();
        int[] postOrderArray = postOrder.stream().mapToInt(Integer::intValue).toArray();
        return new int[][]{preOrderArray, inOrderArray, postOrderArray};
    }

    public void preOrder(TreeNode root) {
        if(root == null) return;
        preOrder.add(root.val);
        preOrder(root.left);
        preOrder(root.right);
    }

    public void inOrder(TreeNode root) {
        if(root == null) return;
        inOrder(root.left);
        inOrder.add(root.val);
        inOrder(root.right);
    }

    public void postOrder(TreeNode root) {
        if(root == null) return;
        postOrder(root.left);
        postOrder(root.right);
        postOrder.add(root.val);
    }
}