前缀、中缀、后缀表达式

#include <iostream>
#include <stack>
using namespace std;
double evaluate(double n1, char c, double n2)
{
	switch (c)
	{
	case ‘+‘: return n1 + n2;
	case ‘-‘: return n1 - n2;
	case ‘*‘: return n1 * n2;
	case ‘/‘: return n1 / n2;
	}
}
int main(void)
{
	//解析前缀
	char expr1[] = "-*+1234";
	stack<double> sta;
	int i = strlen(expr1) - 1;
	int num1, num2;
	char c;
	do
	{
		c = expr1[i];
		if (isdigit(c)) sta.push(c - ‘0‘);
		else
		{
			num1 = sta.top();
			sta.pop();
			num2 = sta.top();
			sta.pop();
			sta.push(evaluate(num1, c, num2));
		}
		i--;
	} while (i > -1);
	cout << sta.top() << endl;
	sta.pop();
	//解析后缀
	char expr2[] = "12+3*4-";
	int n = strlen(expr2);
	i = 0;
	do
	{
		c = expr2[i];
		if (isdigit(c)) sta.push(c - ‘0‘);
		else
		{
			num1 = sta.top();
			sta.pop();
			num2 = sta.top();
			sta.pop();
			sta.push(evaluate(num2, c, num1));
		}
		i++;
	} while (i < n);
	cout << sta.top() << endl;
	return 0;
}

char ops[] = "+-*/";
int get_op(char c)
{
	for (int i = 0; i < 4; i++)
		if (c == ops[i]) return i;
	return -1;
}
/*
  +  -  *  /  
+ 0  0 -1 -1  
- 0  0 -1 -1  
* 1  1  0  0  
/ 1  1  0  0  
*/
int priority[4][4]=
{
	{ 0, 0, -1, -1 },
	{ 0, 0, -1, -1 },
	{ 1, 1, 0, 0 },
	{ 1, 1, 0, 0 }
};
int main(void)
{
	//中缀转前缀
	char expr[] = "1+((2+3)*4)-5";
	stack<char> s1, s2;
	int i, n;
	char c;
	i = strlen(expr) - 1;  //从右向左扫描
	do
	{
		c = expr[i];
		if (isdigit(c)) s2.push(c);  //数字直接压栈s2
		else
		{
			if (‘)‘ == c) s1.push(c);  //遇到右括号，直接压入栈s1
			else if (‘(‘ == c)   //遇到左括号
			{
				while (‘)‘ != s1.top())
				{
					s2.push(s1.top());
					s1.pop();
				}
				s1.pop();
			}
			else  //其它非括号运算符
			{
				if (s1.empty() || ‘)‘ == s1.top()) s1.push(c);
				else
				{
					while (!s1.empty() && (-1 == priority[get_op(c)][get_op(s1.top())]))
					{
						s2.push(s1.top());
						s1.pop();
					}
					s1.push(c);
				}
			}
		}
		i--;
	} while (i > -1);
	while (!s1.empty())   //把s1中剩余字符压入s2
	{
		s2.push(s1.top());
		s1.pop();
	}
	do
	{
		cout << s2.top();
		s2.pop();
	} while (!s2.empty());
	return 0;
}

int main(void)
{
	//中缀转后缀
	char expr[] = "1+((2+3)*4)-5";
	stack<char> s1, s2;
	int i, n;
	char c;
	i = 0;   //从左至右扫描中缀表达式
	n = strlen(expr);
	do
	{
		c = expr[i];
		if (isdigit(c)) s2.push(c);  //数字直接压栈s2
		else
		{
			if (‘(‘ == c) s1.push(c);   //左括号
			else if (‘)‘ == c)    //右括号
			{
				while (‘(‘ != s1.top())
				{
					s2.push(s1.top());
					s1.pop();
				}
				s1.pop();
			}
			else
			{
				if (s1.empty() || ‘(‘ == s1.top()) s1.push(c);
				else
				{
					while (!s1.empty() && (priority[get_op(c)][get_op(s1.top())]) <= 0)
					{
						s2.push(s1.top());
						s1.pop();
					}
					s1.push(c);
				}
			}
		}
		i++;
	} while (i < n);
	while (!s2.empty())   //把s1中剩余字符压入s2
	{
		s1.push(s2.top());
		s2.pop();
	}
	do
	{
		cout << s1.top();
		s1.pop();
	} while (!s1.empty());
	return 0;
}

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