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> result = new ArrayList<Integer>();
    public int[] preorderTraversal (TreeNode root) {
        // write code here
        if (root!=null){
            System.out.println(root.val);
            result.add(root.val);
            preorderTraversal(root.left);
            preorderTraversal(root.right);
        }

        int[] ret = new int[result.size()];
        for (int i=0;i<result.size();i++){
            ret[i]= result.get(i);
        }
        return ret;
     }
}