Java：树的前序、中序、后序遍历代码实现--利用栈实现

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前言

树的前序、中序、后序遍历是常见的树检索方式，本文分享了Java实现树的前序、中序、后序遍历的代码–利用栈实现

希望对大家有用！

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一、具体实现代码

1.前序遍历

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
        List<Integer> list=new ArrayList<>();
        Stack<TreeNode> s = new Stack<TreeNode>();
        if(root == null)
            return  new int[0];
        s.push(root);
        while(!s.isEmpty()){
            TreeNode node=s.pop();
            list.add(node.val);
            if(node.right!=null){
                s.push(node.right);
            }
            if(node.left!=null){
                s.push(node.left);
            }
        }
        int[] res = new int[list.size()];
        for(int i=0;i<list.size();i++){
            res[i]=list.get(i);
        }
        return res;
    }
}

2.中序遍历

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 {
    public int[] inorderTraversal (TreeNode root) {
        //添加遍历结果的数组
        List<Integer> list = new ArrayList();
        Stack<TreeNode> s = new Stack<TreeNode>();
        //空树返回空数组
        if(root == null)
            return new int[0];
        //当树节点不为空或栈中有节点时
        while(root != null || !s.isEmpty()){
            //每次找到最左节点
            while(root != null){
                s.push(root);
                root = root.left;
            }
            //访问该节点
            TreeNode node = s.pop();
            list.add(node.val);
            //进入右节点
            root = node.right;
        }
        //返回的结果
        int[] res = new int[list.size()];
        for(int i = 0; i < list.size(); i++)
            res[i] = list.get(i);
        return res;
    }
}

3.后序遍历

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 {
    public int[] postorderTraversal (TreeNode root) {
        //添加遍历结果的数组
        List<Integer> list = new ArrayList();
        Stack<TreeNode> s = new Stack<TreeNode>();
        TreeNode pre = null;
        while(root != null || !s.isEmpty()){
            //每次先找到最左边的节点
            while(root != null){
                s.push(root);
                root = root.left;
            }
            //弹出栈顶
            TreeNode node = s.pop();
            //如果该元素的右边没有或是已经访问过
            if(node.right == null || node.right == pre){
                //访问中间的节点
                list.add(node.val);
                //且记录为访问过了
                pre = node;
            }else{
                //该节点入栈
                s.push(node);
                //先访问右边
                root = node.right;
            }
        }
        //返回的结果
        int[] res = new int[list.size()];
        for(int i = 0; i < list.size(); i++)
            res[i] = list.get(i);
        return res;
    }
}

更多内容可以参考：BM23 二叉树的前序遍历