2021-11-09

二叉树的几种常见的遍历java实现。

package day01.binarytree;

import java.util.Arrays;
import java.util.LinkedList;
import java.util.Stack;

public class BinaryTree {

    //构建二叉数节点
    private static class TreeNode {
        int data;
        TreeNode leftchild;
        TreeNode rightchild;
        TreeNode(int data) {
            this.data = data;
        }
    }
    //创建二叉树
    //利用LinkedList保存节点的信息值
    public static TreeNode createTreeNode(LinkedList<Integer> inputlinked) {
        if (inputlinked == null|| inputlinked.isEmpty()) return null;
        TreeNode node = null;
        //取出第一个元素并移出
        Integer data = inputlinked.removeFirst();
        if (data != null) {
             node = new TreeNode(data);
            //运用递归进行建立
            node.leftchild = createTreeNode(inputlinked);
            node.rightchild = createTreeNode(inputlinked);
        }
        return node;
    }
    //先序遍历二叉树
    public static void preOrderTraveral(TreeNode node) {
        if (node == null) return;
        System.out.print(node.data+" ");
        //遍历左子树
        preOrderTraveral(node.leftchild);
        //遍历右子树
        preOrderTraveral(node.rightchild);
    }
    //中序遍历
    public static void inOrderTraveral(TreeNode node) {
        if (node == null) return;
        //先遍历左子树
        inOrderTraveral(node.leftchild);
        System.out.print(node.data+" ");
        inOrderTraveral(node.rightchild);
    }
    //后序遍历
    public static void postOrderTraveral(TreeNode node) {
        if (node == null) return;
        //先遍历左子树
        postOrderTraveral(node.leftchild);
        //遍历右子树
        postOrderTraveral(node.rightchild);
        System.out.print(node.data+" ");
    }
    //非递归前序遍历借用栈来实现
    public static void preOrderTraveralWithStack(TreeNode root) {
        //建立一个栈
        Stack<TreeNode> stack = new Stack<>();
        TreeNode treeNode = root;
        //如果节点不为空 就让节点ru
        while (treeNode != null || !stack.isEmpty()) {
            while (treeNode != null) {
                //访问孩子节点并入栈
                System.out.print(treeNode.data+" ");
                stack.push(treeNode);
                treeNode = treeNode.leftchild;
            }
            if (!stack.isEmpty()) {
                //弹出栈顶元素
                treeNode=stack.pop();
                treeNode = treeNode.rightchild;
            }


        }
    }

    public static void main(String[] args) {
    //运用LinkedList来存储二叉树所有的数据
        LinkedList<Integer> inputList = new LinkedList<>(Arrays.asList(new Integer[]{3,2,9,null,null,10,null,null,8,null,4}));
//        System.out.println(inputList.toString());
        TreeNode treeNode = createTreeNode(inputList);
        System.out.print("先序遍历:");
        preOrderTraveral(treeNode);
        System.out.println();
        System.out.print("中序遍历:");
        inOrderTraveral(treeNode);
        System.out.println();
        System.out.print("后序遍历:");
        postOrderTraveral(treeNode);
        System.out.println(" ");
        System.out.print("非递归前序遍历");
        preOrderTraveralWithStack(treeNode);
    }
}

代码测试结果：