[数据结构与算法]144. 二叉树的前序遍历

https://leetcode-cn.com/problems/binary-tree-preorder-traversal/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        preorder(root,res);
        System.out.print(res);
        return res;
    }
    //确定递归函数的参数和返回值
    public void preorder(TreeNode root,List<Integer> res){
        //确定终止条件
        if(root==null){
            return;
        }
        //确定单层递归的逻辑
        res.add(root.val);
        System.out.print(res);
        preorder(root.left,res);
        preorder(root.right,res);
    }
}

https://programmercarl.com/%E4%BA%8C%E5%8F%89%E6%A0%91%E7%9A%84%E9%80%92%E5%BD%92%E9%81%8D%E5%8E%86.html