added evaluation of postfix expression using stack

stack_problems/evaluate_postfix_expression.cpp

@@ -0,0 +1,56 @@
+//Program to evaluate postfix expression
+#include<bits/stdc++.h>
+using namespace std;
+int isOperator(char x){
+    if(x=='+'||x=='-'||x=='*'||x=='/')
+       return 1;
+    else
+       return 0;
+}
+int main() {
+	int T;//Number of expressions you want to evaluate
+	int R=0,result;
+	cin>>T;
+	for(int i=0;i<T;i++){
+	    string s;
+	    cin>>s;
+	    vector<char>sp;
+	    for(int j=0;s[j]!='\0';j++){
+
+	        if(isOperator(s[j])){
+	           int r=(int)sp.back();
+	           sp.pop_back();
+	           int l=(int)sp.back();
+	           sp.pop_back();
+	           switch(s[j]){
+	               case '+':R=l+r;
+				        break;
+	               case '-':R=l-r;
+				   break;
+	               case '*':R=l*r;
+				   break;
+	               case '/':R=l/r;
+				   break;
+
+	           }
+	           sp.push_back(R);
+
+	        }
+	        else{
+	            sp.push_back(s[j]-'0');
+	        }
+	    }
+	    result=sp.back();
+	    cout<<result<<"\n";
+	}
+	return 0;
+}
+
+//example 
+//Input:
+//2
+//231*+9-
+//123+*8-
+//Output:
+//-4
+//-3