牛客在线编程101-23二叉树的前序/中序/后续遍历

た入场券 2023-10-04 19:33 8阅读 0赞

### 题目 ###

给你二叉树的根节点 root ，返回它节点值的 前序/中序/后续 遍历。  
基础题目，需要立即写出答案的题

### 解析 ###

前序

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
            ArrayList<Integer> list = new ArrayList<>();
            preorder(list, root);
            int[] res = new int[list.size()];
            for (int i = 0; i < list.size(); i++) {
        
                res[i] = list.get(i);
            }
            return res;
        }
    
        public void preorder(ArrayList<Integer> res, TreeNode root) {
        
            if (root == null) {
        
                return;
            }
            res.add(root.val);
            preorder(res, root.left);
            preorder(res, root.right);
        }
    }

中序

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[] inorderTraversal (TreeNode root) {
        
            // write code here
            ArrayList<Integer> list = new ArrayList<>();
            inorder(list, root);
            int[] res = new int[list.size()];
            for (int i = 0; i < list.size(); i++) {
        
                res[i]= list.get(i);
            }
            return res;
        }
    
        public void inorder(ArrayList<Integer> list, TreeNode root) {
        
            if (root == null) {
        
                return;
            }
            inorder(list, root.left);
            list.add(root.val);
            inorder(list, root.right);
        }
    }

后续

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[] postorderTraversal (TreeNode root) {
        
            // write code here
            ArrayList<Integer> list = new ArrayList<>();
            postorder(list, root);
            int[] res = new int[list.size()];
            for (int i = 0; i < list.size(); i++) {
        
                res[i] = list.get(i);
            }
            return res;
        }
    
        public void postorder(ArrayList<Integer> list, TreeNode root) {
        
            if (root == null) {
        
                return;
            }
            postorder(list, root.left);
            postorder(list, root.right);
            list.add(root.val);
        }
    }