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) {
        // write code here
        ArrayList<Integer> result = new ArrayList<>();
        preorder(root,result);
        
        int[] arr = new int[result.size()];
        for (int i = 0; i < result.size(); i++) {
            arr[i] = result.get(i);
        }
        return arr;
    }

    private void preorder(TreeNode node, ArrayList<Integer> result) {
        if (node == null) {
            return;
        }
        result.add(node.val);
        preorder(node.left, result);
        preorder(node.right, result);
    }
}

递归是实现前序遍历的最直观方法。递归函数将首先访问根节点，然后递归地访问左子树和右子树。