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
     */
    ArrayList<Integer> arr = new ArrayList<>();

    public int[] preorderTraversal (TreeNode root) {
        if(root != null)
            traversal(root);
        int[] a = new int[arr.size()];
        for(int i = 0; i < a.length; i++) {
            a[i] = arr.get(i);
        }
        return a;
    }

    private void traversal(TreeNode root) {
        arr.add(root.val);
        if(root.left != null)
            preorderTraversal(root.left);

        if(root.right != null)
            preorderTraversal(root.right);

    }
}