infix to postfix.

cpp/infix_to_postfix_with_REVPOL.cpp

@@ -0,0 +1,162 @@
+//infix to postfix conversion using REVPOL algorithm.
+//time complexity of these is O(n^2).
+#include <bits/stdc++.h>
+using namespace std;
+
+struct Node
+{
+    char data;
+    struct Node *link;
+};
+
+struct Node *top;
+
+void push(char data)
+{
+
+    struct Node *temp;
+    temp = new Node();
+    if (!temp)
+    {
+        cout << "\nStack Overflow";
+        exit(1);
+    }
+
+    temp->data = data;
+
+    temp->link = top;
+
+    top = temp;
+}
+char pop()
+{
+    struct Node *temp;
+
+    if (top == NULL)
+    {
+        cout << "Stack Underflow" << endl;
+        return NULL;
+    }
+    else
+    {
+
+        temp = top;
+
+        top = top->link;
+
+        temp->link = NULL;
+        char c = temp->data;
+        free(temp);
+        return c;
+    }
+}
+int in_pre(char c)
+{
+    //giveng priorites to all charecters which are in input string.
+    if (c == '+' || c == '-')
+    {
+        return 1;
+    }
+    else if (c == '*' || c == '/')
+    {
+        return 3;
+    }
+    else if (c == '^')
+    {
+        return 6;
+    }
+    else if (c == '(')
+    {
+        return 9;
+    }
+    else if (c == ')')
+    {
+        return 0;
+    }
+    else
+    {
+        return 7;
+    }
+}
+int stk_pre(char c)
+{
+    //giving priorites to all other characters which are present in stack.
+    if (c == '+' || c == '-')
+    {
+        return 2;
+    }
+    else if (c == '*' || c == '/')
+    {
+        return 4;
+    }
+    else if (c == '^')
+    {
+        return 5;
+    }
+    else if (c == '(')
+    {
+        return 0;
+    }
+    else
+    {
+        return 8;
+    }
+}
+int count_rank(char c)
+{
+    //counting rank of output postfix expression.
+    if (c == '+' || c == '-' || c == '/' || c == '*' || c == '^')
+    {
+        return -1;
+    }
+    else
+    {
+        return 1;
+    }
+}
+
+int main()
+{
+    string s, post = "";
+    cin >> s;
+    vector<char> v;
+    int tp = 0, rank1 = 0;
+    //initialize stack with '('.
+    v.push_back('(');
+    char ch1;
+    bool flag1 = true;
+    int i = 0;
+    while (s[i])
+    {
+        while (in_pre(s[i]) < stk_pre(v[tp]))
+        {
+            ch1 = v[tp];
+            post += ch1;
+            rank1 += count_rank(ch1);
+            v.pop_back();
+            tp--;
+            if (rank1 < 1)
+            {
+                cout << "invalid infix" << endl;
+                flag1 = false;
+                break;
+            }
+        }
+        if (flag1 && (stk_pre(v[tp]) != in_pre(s[i])))
+        {
+            v.push_back(s[i]);
+            tp++;
+        }
+        else if (flag1)
+        {
+            v.pop_back();
+            tp--;
+        }
+        i++;
+    }
+    if (rank1 == 1 && tp == -1)
+    {
+        cout << post << endl;
+    }
+    return 0;
+}