面试必刷TOP101：23、二叉树的前序遍历

题目

题解

import java.util.*;
public class Solution {
    public void preorder(List<Integer> list, TreeNode root){
        //遇到空节点则返回
        if(root == null) 
            return;
        //先遍历根节点
        list.add(root.val); 
        //再去左子树
        preorder(list, root.left); 
        //最后去右子树
        preorder(list, root.right); 
    }
    
    public int[] preorderTraversal (TreeNode root) {
        //添加遍历结果的数组
        List<Integer> list = new ArrayList(); 
        //递归前序遍历
        preorder(list, root); 
        //返回的结果
        int[] res = new int[list.size()]; 
        for(int i = 0; i < list.size(); i++)
            res[i] = list.get(i);
        return res;
    }
}