Merge pull request rathoresrikant#559 from shashikant95/master

Update Data Structures/Stacks/infix_to_postfix.cpp

Data Structures/Stacks/infix_to_postfix.cpp

@@ -0,0 +1,103 @@
+#include <bits/stdc++.h>
+using namespace std;
+struct Stack
+{
+    int top;
+    int capacity;
+    int *arr;
+};
+Stack* createStack(unsigned int capacity)
+{
+    Stack *stack = new Stack;
+    if(!stack)
+    return NULL;
+    stack->top = -1;
+    stack->capacity = capacity;
+    stack->arr = new int[stack->capacity];
+    if(!stack->arr)
+    return NULL;
+    return stack;
+}
+int isEmpty(Stack* stack)
+{
+    return stack->top == -1;
+}
+char peek(Stack* stack)
+{
+    return stack->arr[stack->top];
+}
+char pop(Stack* stack)
+{
+    if(!isEmpty(stack))
+    return stack->arr[stack->top--];
+    return '$';
+}
+void push(Stack *stack, char op)
+{
+    stack->arr[++stack->top] = op;
+}
+int size(Stack* stack)
+{
+    return stack->top + 1;
+}
+int isOperand(char ch)
+{
+    return (ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z');
+}
+int Prec(char ch)
+{
+    switch(ch)
+    {
+        case '+':
+        case '-':
+        return 1;
+        case '*':
+        case '/':
+        return 2;
+        case '^':
+        return 3;
+        default:
+        return -1;
+    }
+}
+string infixToPostfix(string exp)
+{
+    int i,k;
+    Stack* stack = createStack(exp.length());
+    if(!stack)
+    return NULL;
+    for(i=0,k=-1;exp[i];++i)
+    {
+        if(isOperand(exp[i]))
+        exp[++k] = exp[i];
+        else
+        if(exp[i]=='(')
+        push(stack,exp[i]);
+        else
+        if(exp[i]==')')
+        {
+            while (!isEmpty(stack) && peek(stack) != '(')
+                exp[++k] = pop(stack);
+            if (!isEmpty(stack) && peek(stack) != '(')
+                return NULL;         
+            else
+                pop(stack);
+        }
+        else
+        {
+            while(!isEmpty(stack) && Prec(exp[i] <= Prec(peek(stack))))
+            exp[++k] = pop(stack);
+            push(stack,exp[i]);
+        }
+    }
+    while(!isEmpty(stack))
+        exp[++k] = pop(stack);
+    return exp;
+}
+int main()
+{
+    Stack* stack = createStack(10);
+    string exp = "a+b*(c^d-e)^(f+g*h)-i";
+    cout<<infixToPostfix(exp);
+    return 0;
+}