C++ 数据机构转化

A + B
A + B-C
(A + B) + (C-D)

A B +   
A B + C-  
A B C * +
A B + C * D-

#include<iostream>
#include<stack>
using namespace std;
// defines the boolean function for operator, operand, equalOrhigher precedence and the string conversion function.
bool IsOperator(char);
bool IsOperand(char);
bool eqlOrhigher(char, char);
string convert(string);
int main()
{
string infix_expression, postfix_expression;
int ch;
do
{
cout << " Enter an infix expression: ";
cin >> infix_expression;
 postfix_expression = convert(infix_expression);
 cout << "\n Your Infix expression is: " << infix_expression;
cout << "\n Postfix expression is: " << postfix_expression;
cout << "\n \t do you want to enter infix expression (1/ 0)?";
cin >> ch;
//cin.ignore(); 
} while(ch == 1);
return 0;
}
// define the IsOperator() function to validate whether any symbol is operator.
/* if the symbol is operator, it returns true, otherwise false. */
bool IsOperator(char c)
{
if(c == '+' || c == '-' || c == '*' || c == '/' || c == '^' )
return true;   
return false;
}
// IsOperand() function is used to validate whether the character is operand.
bool IsOperand(char c)
{
if( c >= 'A' && c <= 'Z')  /* Define the character in between A to Z. if not, it returns false.*/
return true;
if (c >= 'a' && c <= 'z')  // Define the character in between a to z. if not, it returns false. */
return true;
if(c >= '0' && c <= '9')   // Define the character in between 0 to 9. if not, it returns false. */
return true;
return false;
}
// here, precedence() function is used to define the precedence to the operator.
int precedence(char op)
{
if(op == '+' || op == '-')                   /* it defines the lowest precedence */
return 1;
if (op == '*' || op == '/')
return 2;
if(op == '^')                                  /* exponent operator has the highest precedence *
return 3;      
return 0;
}
/* The eqlOrhigher() function is used to check the higher or equal precedence of the two operators in infix expression. */
bool eqlOrhigher (char op1, char op2)
{
int p1 = precedence(op1);
int p2 = precedence(op2);
if (p1 == p2)
{
if (op1 == '^' )
return false;
return true;
}
return  (p1>p2 ? true : false);
}
/* string convert() function is used to convert the infix expression to the postfix expression of the Stack */
string convert(string infix)
{
stack <char> S;
string postfix ="";  
char ch;
S.push( '(' );
infix += ')';
for(int i = 0; i<infix.length(); i++)
{
ch = infix[i];
if(ch == ' ')
continue;
else if(ch == '(')
S.push(ch);
else if(IsOperand(ch))
postfix += ch;
else if(IsOperator(ch))
{
while(!S.empty() && eqlOrhigher(S.top(), ch))
{
postfix += S.top();
S.pop();
}
S.push(ch);
}
else if(ch == ')')
{
while(!S.empty() && S.top() != '(')
{
postfix += S.top();
S.pop();
}
S.pop();
}
}
return postfix;
}