java 二维List转二维数组

import java.util.*;

public class Solution {

List<Integer> list = new ArrayList<>();
private List<List<Integer>> result = new ArrayList<>();

public int[][] threeOrders (TreeNode root) {
    // 先遍历二叉树，并添加到 result 中
    preOrder(root);
    result.add(new ArrayList(list));
    list.clear();
    inOrder(root);
    result.add(new ArrayList(list));
    list.clear();
    postOrder(root);
    result.add(new ArrayList(list));
    // 将二层 List 转为 二维数组
    int[][] res = new int[result.size()][result.get(0).size()];
    for(int i=0; i<result.size(); i++) {
        res[i] = result.get(i).stream().mapToInt(Integer::valueOf).toArray();
    }
    return res;
}

// 后面三种二叉树的递归遍历

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

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

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

}