144. Preorder traversal of binary tree

144. Preorder traversal of two tree

Ideas ：
In general, what binary tree pays attention to is a complete solution system and framework routine , Not many people really understand , Most people will be confused by the program during recursive traversal . But in fact, as long as you understand the difference between loop and recursion , Even if you understand the binary tree .

This problem is the preorder traversal of binary tree , On the force button, it is a simple question , So readers will probably understand my code and see the problem-solving framework .

The topic is to let the preorder traverse , Enter a binary tree , You output its preorder traversal result . This problem is nothing more than writing a traversal array (travser) Traverse the binary tree alone , At the same time, operate on the key nodes of traversal （ Preorder traversal position / Middle order traversal position / Postorder traversal position ）, For example, this problem . When traversing to the preamble position , You should save the corresponding value of the node at this time into an array , Here I define a Arraylist Save the results .

/** * 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 {
    
    
    ArrayList<Integer> res = new ArrayList<>();


    public List<Integer> preorderTraversal(TreeNode root) {
    
        travser(root);
        return res;
    }

    void travser(TreeNode node){
    
        if(node == null){
    
            return ;
        }
        res.add(node.val);
        travser(node.left);
        travser(node.right);
    }
}