回顾：树的几种迭代遍历方式

Posted 2022-10-18 boy_nihao

PS：文章最后附上完整代码

一、中序遍历

List<Integer> inOrder(BTree root) 
        List<Integer> result = new ArrayList<>();

        Stack<BTree> stack = new Stack<>();
        BTree node;
        while (null != root || !stack.empty()) 
            while (null != root) 
                stack.push(root);
                root = root.left;
            
            node = stack.pop();
            result.add(node.val);
            if (null != node.right) 
                root = node.right;
            
        

        return result;
    

二、前序遍历

List<Integer> preOrder(BTree root) 
        List<Integer> result = new ArrayList<>();

        Stack<BTree> stack = new Stack<>();
        BTree node;
        while (null != root || !stack.empty()) 
            while (null != root) 
                result.add(root.val);
                stack.push(root);
                root = root.left;
            
            node = stack.pop();
            if (null != node.right) 
                root = node.right;
            
        

        return result;
    

三、后序遍历

List<Integer> postOrder(BTree root) 
        List<Integer> result = new ArrayList<>();

        Stack<BTree> stack = new Stack<>();
        BTree node;
        while (null != root || !stack.empty()) 
            while (null != root) 
                result.add(0, root.val);
                stack.push(root);
                root = root.right;
            
            node = stack.pop();
            if (null != node.left) 
                root = node.left;
            
        

        return result;
    

四、层次遍历

List<Integer> layerOrder(BTree root) 
        List<Integer> result = new ArrayList<>();
        if (null == root) 
            return result;
        

        Queue<BTree> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) 
            BTree node = queue.poll();
            if (null != node.left) 
                queue.offer(node.left);
            
            if (null != node.right) 
                queue.offer(node.right);
            
            result.add(node.val);
        

        return result;
    

测试代码：

import java.util.*;

/**
 * Copyright © 2018 Chris. All rights reserved.
 *
 * @author Chris
 */

public class Solution 

    static class BTree 
        int val;
        BTree left, right;
        BTree(int v) val = v;

        @Override
        public String toString() 
            return val + "";
        
    

    /**
     * 层次遍历
     *
     * @param root 根结点
     * @return 遍历结果
     */
    List<Integer> layerOrder(BTree root) 
        List<Integer> result = new ArrayList<>();
        if (null == root) 
            return result;
        

        Queue<BTree> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) 
            BTree node = queue.poll();
            if (null != node.left) 
                queue.offer(node.left);
            
            if (null != node.right) 
                queue.offer(node.right);
            
            result.add(node.val);
        

        return result;
    
    /**
     * 后序遍历
     *
     * @param root 根结点
     * @return 遍历结果
     */

    List<Integer> postOrder(BTree root) 
        List<Integer> result = new ArrayList<>();

        Stack<BTree> stack = new Stack<>();
        BTree node;
        while (null != root || !stack.empty()) 
            while (null != root) 
                result.add(0, root.val);
                stack.push(root);
                root = root.right;
            
            node = stack.pop();
            if (null != node.left) 
                root = node.left;
            
        

        return result;
    

    /**
     * 前序遍历
     *
     * @param root 根结点
     * @return 遍历结果
     */
    List<Integer> preOrder(BTree root) 
        List<Integer> result = new ArrayList<>();

        Stack<BTree> stack = new Stack<>();
        BTree node;
        while (null != root || !stack.empty()) 
            while (null != root) 
                result.add(root.val);
                stack.push(root);
                root = root.left;
            
            node = stack.pop();
            if (null != node.right) 
                root = node.right;
            
        

        return result;
    

    /**
     * 中序遍历
     *
     * @param root 根结点
     * @return 遍历结果
     */
    List<Integer> inOrder(BTree root) 
        List<Integer> result = new ArrayList<>();

        Stack<BTree> stack = new Stack<>();
        BTree node;
        while (null != root || !stack.empty()) 
            while (null != root) 
                stack.push(root);
                root = root.left;
            
            node = stack.pop();
            result.add(node.val);
            if (null != node.right) 
                root = node.right;
            
        

        return result;
    

    public static void main(String args[]) 

        Solution s = new Solution();
        BTree root = new BTree(1);
        BTree left = new BTree(2);
        BTree right = new BTree(3);
        left.left = new BTree(4);
        left.right = new BTree(5);
        right.left = new BTree(6);
        right.right = new BTree(7);
        root.left = left;
        root.right  = right;
        System.out.println(s.inOrder(root));
        System.out.println(s.preOrder(root));
        System.out.println(s.postOrder(root));
        System.out.println(s.layerOrder(root));