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
        List<Integer> list = new ArrayList<>();
        preorder(list, root);
        int it[] = new int[list.size()];
        for (int i = 0 ; i < list.size(); i++ ) {
            it[i] = list.get(i);
        }
        return it;
    }


    private void preorder(List<Integer> list, TreeNode root) {
        if (root == null) {
            return ;
        }
        list.add(root.val);
        // 遍历左节点
        preorder(list, root.left);
        // 遍历右节点
        preorder(list, root.right);
    }
}

前序遍历是DLR，递归，退出节点是节点为null