从后缀表达式建表达式树【二叉树】

伪代码

FOR each char in string:
	IF char is num :
		new node(num), then push ptr into stack.
        ELSE IF char is Ops:
        	get top-2 of stack as l、r,
                then new node(Ops,l,r),
                push ptr into stack.
    	ELSE:
        	print error-info, then exit.
RETURN the top of stack, which is root of tree.

过程

c++实现

#pragma once

#include<iostream>
#include<stack>
#include<cstring>

using namespace std;


typedef char Elem;

//binary tree node abstract class
template<class Elem>
class BinNode {
public:
	virtual Elem& val() = 0;
	virtual void setVal(const Elem&) = 0;
	virtual BinNode* left() const = 0;
	virtual void setLeft(BinNode*) = 0;
	virtual BinNode* right() const = 0;
	virtual void setRight(BinNode*) = 0;
	virtual bool isLeaf() = 0;
};

template<class Elem>
class BinNodePtr : public BinNode<Elem> {
private:
	Elem value;
	BinNodePtr* lc;
	BinNodePtr* rc;//right child
public:
	BinNodePtr() :value(0), lc(nullptr), rc(nullptr) {}
	BinNodePtr(Elem e, BinNodePtr* l = nullptr, BinNodePtr* r = nullptr) :value(e), lc(l), rc(r) {}
	~BinNodePtr() {}
	//func
	Elem& val() { return value; }
	void setVal(const Elem& e) { value = e; }

	inline BinNode<Elem>* left() const { return lc; }
	void setLeft(BinNode<Elem>* p) { lc = (BinNodePtr*)p; }
	inline BinNode<Elem>* right() const { return rc; }
	void setRight(BinNode<Elem>* p) { rc = (BinNodePtr*)p; }

	bool isLeaf() {
		return (rc == nullptr) && (lc == nullptr);
	}
};


template<class Elem>
BinNodePtr<Elem>* createNode(const Elem &item, BinNodePtr<Elem>* l=nullptr, BinNodePtr<Elem>* r=nullptr) {
	return new BinNodePtr<Elem>(item,l,r);
}

bool isOps(char ch) {
	return ch == '+' || ch == '-' || ch == '*' || ch == '/';
}


BinNodePtr<Elem>* fromPostfixExprCreateBiTree(const char* str) {
	stack<BinNodePtr<Elem>*> stk;
	int len = strlen(str);

	for (int i = 0; i < len; i++)
	{
		char ch = str[i];
		if (ch >= 'a' && ch <= 'z') {
			stk.push(createNode(ch));
		}
		else if (isOps(ch)) {
			if (stk.empty()) {
				printf_s("str not right!!\n");
				exit(-1);
			}
			BinNodePtr<Elem>* r = stk.top(); stk.pop();
			BinNodePtr<Elem>* l = stk.top(); stk.pop();
			stk.push(createNode(ch, l, r));
		}
		else {
			printf_s("str not right!!\n");
			exit(-1);
		}
	}

	if (stk.size() != 1) {
		printf_s("str not right!!\n");
		exit(-1);
	}

	return stk.top();
}



void preVisit(BinNode<Elem>* root) {
	if (root) {
		printf_s("%c  ", root->val());
		preVisit(root->left());
		preVisit(root->right());
	}
}


void testFunc() {
	char str[] = "ab+cde+**";
	preVisit(fromPostfixExprCreateBiTree(str));
}