Postfix to Infix

PostfixToInfix.cpp

#include <string>
#include <stack>
#include <iostream>
using namespace std;

bool isOperand(char x)
{
	return (x >= 'a' && x <= 'z') ||
		(x >= 'A' && x <= 'Z');
}

string getInfix(string exp)
{
	stack<string> s;
	stack<int> level;//  */ is one, +- is 0, only character is -1

	for (auto i = 0; exp[i] != '\0'; i++)
	{
		if (isOperand(exp[i]))
		{
			string op(1, exp[i]);
			s.push(op);
			level.push(-1);
		}
		else
		{
			auto op1 = s.top();
			s.pop();
			auto op2 = s.top();
			s.pop();

			auto l1 = level.top();
			level.pop();
			auto l2 = level.top();
			level.pop();

			/*
			 * Listed situations of l1 and l2:
			 *    -1  0  1
			 * -1  1  4  3
			 *  0  4  1  2
			 *  1  3  2  1
			 */

			if (l1 == l2)//Situation 1
			{
				s.push(op2 + exp[i] + op1);

				if (exp[i] == '*' || exp[i] == '/')
					level.push(1);
				else
					level.push(0);
			}
			else if (l1 >= 0 && l2 >= 0)//Situation 2
			{
				if (exp[i] == '*' || exp[i] == '/')
				{
					if (l1 == 0)
						s.push(op2 + exp[i] + "(" + op1 + ")");
					else
						s.push("(" + op2 + ")" + exp[i] + op1);
					level.push(1);
				}
				else
				{
					s.push(op2 + exp[i] + op1);
					level.push(0);
				}

			}
			else if (l1 == 1 || l2 == 1)//Situation 3
			{
				s.push(op2 + exp[i] + op1);
				if (exp[i] == '*' || exp[i] == '/')
					level.push(1);
				else
					level.push(0);
			}
			else//Situation 4
			{

				if (exp[i] == '*' || exp[i] == '/')
				{
					if (l1 == -1)
						s.push("(" + op2 + ")" + exp[i] + op1);
					else
						s.push(op2 + exp[i] + "(" + op1 + ")");

					level.push(1);
				}
				else
				{
					s.push(op2 + exp[i] + op1);
					level.push(0);
				}
			}

		}
	}

	return s.top();
}

int main()
{
	string exp = "ab+c*";
	cout << getInfix(exp) << endl;

	exp = "abc*+de*f+g*+";
	cout << getInfix(exp) << endl;

	exp = "ab+cde+**";
	cout << getInfix(exp) << endl;
	
	system("pause");
	return 0;
}