chore(CPlusPlus): add prefix to postfix stacks (#674)

Co-authored-by: Ming Tsai <37890026+ming-tsai@users.noreply.github.com>
Co-authored-by: Rahul Rajeev Pillai <66192267+Ashborn-SM@users.noreply.github.com>

algorithms/CPlusPlus/README.md

@@ -88,6 +88,7 @@
 - [Stack using Array](Stacks/stack-using-array.cpp)
 - [Infix to postfix expression conversion](Stacks/infix-to-postfix.cpp)
 - [Stock Span Problem using Stacks](Stacks/stock-span-problem.cpp)
+- [Prefix to Postfix expression conversion](Stacks/prefix_to_postfix.cpp)
 
 ## Sorting
 

algorithms/CPlusPlus/Stacks/prefix_to_postfix.cpp

@@ -0,0 +1,64 @@
+// Program to convert a prefix expression to postfix expression
+// Prefix expression : An expression in which the operator appears before the operands. 
+// Postfix expression : An expression in which the operator appears after the operands. 
+
+
+
+// Alogorithm
+// To convert prefix to postfix expression we follow steps given below:
+// Step 1. If the symbol is an operand, push it to the stack
+// Step 2. If symbol is an operator then pop top two elements from stack
+// Step 3. Then push an expression by concatenating (+op1+op2+symbol)
+// Step 4. At last return the top element of stack as postfix expression
+
+
+
+// CODE:
+#include<bits/stdc++.h>
+using namespace std;
+
+// Function to convert prefix to postfix expression
+string prefix_to_postfix(string prefix)
+{
+    stack<string> s;
+    // length of prefix expression
+    int l=prefix.length();
+  
+    // Iterating through the Prefix expression from right to left
+    for(int i=l-1;i>=0;i--){
+        // if the symbol is an operand, push it to the stack
+        if(prefix[i]>='a' && prefix[i]<='z'||prefix[i]>='A' && prefix[i]<='Z'){
+            s.push(string(1,prefix[i]));
+        }
+
+        // if symbol is an operator
+        else
+        { 
+            string op1=s.top();
+            s.pop();
+            string op2=s.top();
+            s.pop();
+            string temp=(op1+op2+prefix[i]);
+            s.push(temp);
+        }
+    }
+    // returning element at top of stack
+    return s.top();
+}
+
+// MAIN FUNCTION
+int main()
+{
+  string exp;
+  cout<<"Enter expression: ";
+  cin>>exp;
+  cout<<"Prefix to Postfix expression is: "<<prefix_to_postfix(exp);
+}
+
+
+// OUTPUT:
+// Enter expression: *-a/bc-/akl
+// Prefix to Postfix expression is: abc/-ak/l-*
+
+
+