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
     */
    List<Integer> list = new ArrayList<>();

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

    public int[] preorderTraversal(TreeNode root) {
        preOrder(root);
        int[] pre = new int[list.size()];
        for (int i = 0; i < pre.length; i++) {
            pre[i] = list.get(i);
        }
        return pre;
    }
}