Leetcode skimming: binary tree 01 (preorder traversal of binary tree)

144. Preorder traversal of two tree

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1. Determine the parameters and return values of recursive functions
2. Determine termination conditions
3. Determine the logic of monolayer recursion

package com.programmercarl.tree;

import java.util.ArrayList;
import java.util.List;

/** * @ClassName PreorderTraversal * @Descriotion TODO * @Author nitaotao * @Date 2022/7/1 15:21 * @Version 1.0 * 144.  Preorder traversal of two tree  * https://leetcode.cn/problems/binary-tree-preorder-traversal/ **/
public class PreorderTraversal {
    
    /** *  The former sequence traversal , Root left and right . * @param root * @return */
    public List<Integer> preorderTraversal(TreeNode root) {
    
        List<Integer> result = new ArrayList<>();
        traversal(root, result);
        return result;
    }

    /** *  The former sequence traversal , Root left and right . * * @param root * @param result * 1. Determine the parameters and return values of recursive functions  * 2. Determine termination conditions  * 3. Determine the logic of monolayer recursion  */
    public  void traversal(TreeNode root, List<Integer> result) {
    
        // Termination conditions 
        if (root == null) {
    
            return;
        }
        // Access root 
        result.add(root.val);
        // Visit the left subtree 
        traversal(root.left, result);
        // Visit the right subtree 
        traversal(root.right,result);
    }
}