java使用栈构造二叉树(不会构造二叉树的看进来)

遍历就很简单了 大家自己遍历遍历就行

感觉题目其实表达的不是特别清楚 我第一次构造树的时候就搞错了

import java.util.*;

public class Main{
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int rootVal = in.nextInt();
        // 使用栈来构造二叉树
        TreeNode root = new TreeNode(rootVal);
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            int fa = in.nextInt();
            int lch = in.nextInt();
            int rch = in.nextInt();
            TreeNode now = stack.pop();
            if(rch != 0){
                TreeNode right = new TreeNode(rch);
                stack.push(right);
                now.right = right;
            }
            if(lch != 0){
                TreeNode left = new TreeNode(lch);
                stack.push(left);
                now.left = left;
            }
        }
        // 调用递归 
        Main app = new Main();
        app.preTraverse(root);
        System.out.println();
        app.nowTraverse(root);
        System.out.println();
        app.afterTraverse(root);
    }

    /*====================================递归=====================================*/
    public void preTraverse(TreeNode root){
        if(root == null) return;
        System.out.print(root.val + " ");
        preTraverse(root.left);
        preTraverse(root.right);
    }

    public void nowTraverse(TreeNode root){
        if(root == null) return;
        nowTraverse(root.left);
        System.out.print(root.val + " ");
        nowTraverse(root.right);
    }

    public void afterTraverse(TreeNode root){
        if(root == null) return;
        afterTraverse(root.left);
        afterTraverse(root.right);
        System.out.print(root.val + " ");
    }



    static class TreeNode{
        public int val;
        public TreeNode left;
        public TreeNode right;

        public TreeNode(int val){
            this.val = val;
        }

        public TreeNode(int val, TreeNode left, TreeNode right){
            this.val = val;
            this.left = left;
            this.right = right;
        }

    }
}