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 int[] preorderTraversal (TreeNode root) {
        List<Integer> result = new ArrayList<>();
        dfs(result, root);
        int[] r = new int[result.size()];
        for(int i=0; i<result.size(); i++) {
            r[i] = result.get(i);
        }
        return r;
    }

    public void dfs(List<Integer> result, TreeNode root) {
        if (root == null) {
            return;
        }
        result.add(root.val);
        dfs(result, root.left);
        dfs(result, root.right);
    }

}