题解 | 二叉树的前序遍历

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 {
    /**
     * 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 
     * @return int整型一维数组
     */
     public void preOrder(List<Integer> list, TreeNode root){
        if(root == null) return;
        list.add(root.val);
        preOrder(list, root.left);
        preOrder(list, root.right);

     }
    public int[] preorderTraversal (TreeNode root) {
        // write code here
        List<Integer> list = new ArrayList<Integer>();
        preOrder(list, root);
        int[] res = list.stream()
                    .mapToInt(Integer::intValue)
                    .toArray();
        return res;
    }
}