构建表达式树-（栈树、中序遍历）

import java.util.Stack;

public class BinaryTreeExpression {
    public static void main(String[] args) {
        String[] str = {"a","b","+","c","d","e","+","*","*"};
        Stack<binaryTree> stack = new Stack<>();    //栈树

        binaryTree t1;
        binaryTree t2;
        binaryTree tree;

        treenode node = new treenode();

        for (String s: str) {
            if (s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")){
                t1 = stack.pop();
                t2 = stack.pop();

                tree = new binaryTree();
                node = new treenode(s);
                tree.setRoot(node);

                node.setLeft(t2.root);
                node.setRight(t1.root);

                stack.push(tree);
            }else {
                tree = new binaryTree();
                tree.setRoot(new treenode(s));
                stack.push(tree);
            }
        }

        tree = stack.peek();    //后缀表达式对应的表达式树

        tree.infixOrder();  //将表达式树展开
    }
}

//定义二叉树
class binaryTree {
    public treenode root;

    public void setRoot(treenode root) {
        this.root = root;
    }


    //定义树的三种遍历
    public void preOrder() {
        if (this.root != null) {
            this.root.preOrder();
        } else {
            System.out.println("当前二叉树为空，无法进行前序遍历");
        }
    }

    public void infixOrder() {
        if (this.root != null) {
            this.root.infixOrder();
        } else {
            System.out.println("当前二叉树为空，无法进行前序遍历");
        }
    }

    public void postOrder() {
        if (this.root != null) {
            this.root.postOrder();
        } else {
            System.out.println("当前二叉树为空，无法进行前序遍历");
        }
    }
}

//构造节点
class treenode {
    private String str;
    private treenode left;
    private treenode right;

    public treenode(String str) {
        this.str = str;
    }

    public treenode() {}

    public void setLeft(treenode left) {
        this.left = left;
    }

    public void setStr(String str) {
        this.str = str;
    }

    public void setRight(treenode right) {
        this.right = right;
    }

    @Override
    public String toString() {
        return "treenode{" +
                "str='" + str + '\'' +
                '}';
    }

    //定义前序遍历
    public void preOrder() {
        System.out.println(this);

        if (this.left != null) {
            this.left.preOrder();
        }

        if(this.right != null) {
            this.right.preOrder();
        }
    }

    //定义中序遍历
    public void infixOrder() {
        if (this.left != null) {
            this.left.infixOrder();
        }

        System.out.println(this);

        if(this.right != null) {
            this.right.infixOrder();
        }
    }

    //定义后序遍历
    public void postOrder() {
        if (this.left != null) {
            this.left.postOrder();
        }

        if(this.right != null) {
            this.right.postOrder();
        }

        System.out.println(this);
    }
}