题解 | #实现二叉树先序，中序和后序遍历# Stream

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    public List<Integer> preTree(TreeNode root) {
        if (root == null) return new ArrayList<Integer>();
        List<Integer> treeList = new ArrayList<>();
        treeList.add(root.val);
        treeList.addAll(preTree(root.left));
        treeList.addAll(preTree(root.right));
        return treeList;
    }
    public List<Integer> midTree(TreeNode root) {
        if (root == null) return new ArrayList<Integer>();
        List<Integer> treeList = new ArrayList<>();
        treeList.addAll(midTree(root.left));
        treeList.add(root.val);
        treeList.addAll(midTree(root.right));
        return treeList;
    }

    public List<Integer> lastTree(TreeNode root) {
        if (root == null) return new ArrayList<Integer>();
        List<Integer> treeList = new ArrayList<>();
        treeList.addAll(lastTree(root.left));

        treeList.addAll(lastTree(root.right));
        treeList.add(root.val);
        return treeList;
    }
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @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
        int result[][] = new int[3][];
        List<Integer> pre = preTree(root);
        int preArr[] = new int[pre.size()];
        preArr = pre.stream().mapToInt(Integer::valueOf).toArray();
        result[0] = preArr;
        List<Integer> mid = midTree(root);
        int midArr[] = new int[mid.size()];
        midArr = mid.stream().mapToInt(Integer::valueOf).toArray();
        result[1] = midArr;
        List<Integer> last = lastTree(root);
        int lastArr[] = new int[last.size()];
        lastArr = last.stream().mapToInt(Integer::valueOf).toArray();
        result[2] = lastArr;
        return result;
    }
}

尽可能利用 Java 提供的标准数据结构，减少自己的代码实现复杂度，不需要自己控制遍历下标。