Evaluate postfix expression using stack in golang

Go program for Evaluate postfix expression using stack. Here problem description and explanation.

package main
import "fmt"
/* 
    Go program for 
    Evaluate postfix expression
*/
// Stack Node
type StackNode struct {
	operand int
	next * StackNode
}
func getStackNode(operand int, 
                  top * StackNode) * StackNode {
	// return new StackNode
	return &StackNode {
		operand,
		top,
	}
}
type MyStack struct {
	top * StackNode
	count int
}
func getMyStack() * MyStack {
	// return new MyStack
	return &MyStack {
		nil,
		0,
	}
}
// Returns the number of element in stack
func(this MyStack) isSize() int {
	return this.count
}
func(this MyStack) isEmpty() bool {
	if this.isSize() > 0 {
		return false
	} else {
		return true
	}
}
// Add a new element in stack
func(this *MyStack) push(value int) {
	// Make a new stack node
	// And set as top
	this.top = getStackNode(value, this.top)
	// Increase node value
	this.count++
}
// Remove a top element 
func(this *MyStack) pop() int {
	var value int = this.peek()
	if this.isEmpty() == false {
		this.top = this.top.next
		// Reduce the size
		this.count--
	}
	return value
}
// Used to get top element of stack
func(this MyStack) peek() int {
	if !this.isEmpty() {
		return this.top.operand
	} else {
		return 0
	}
}

func evaluate(e string) {
	// Get the length of expression
	var size int = len(e)
	var a int = 0
	var b int = 0
	var s * MyStack = getMyStack()
	var isVaild bool = true
	// Work with (+,-,/,*,%) operator
	for i := 0 ; i < size && isVaild ; i++ {
		if e[i] >= '0' && e[i] <= '9' {
			a = int(e[i]) - 48
			s.push(a)
		} else if s.isSize() > 1 {
			a = s.pop()
			b = s.pop()
			// Perform arithmetic operations between 2 operands
			if e[i] == '+' {
				s.push(b + a)
			} else if e[i] == '-' {
				s.push(b - a)
			} else if e[i] == '*' {
				s.push(b * a)
			} else if e[i] == '/' {
				s.push(b / a)
			} else if e[i] == '%' {
				s.push(b % a)
			} else {
				// When use other operator
				isVaild = false
			}
		} else if s.isSize() == 1 {
			// Special case  
			// When use +, - at the beginning
			if e[i] == '-' {
				a = s.pop()
				s.push(-a)
			} else if e[i] != '+' {
				// When not use  +,-
				isVaild = false
			}
		} else {
			isVaild = false
		}
	}
	if isVaild == false {
		// Possible case use other operators
		// 1) When using special operators
		// 2) Or expression is invalid
		fmt.Println(e, " Invalid expression ")
		return
	}
	fmt.Println(e, " = ", s.pop())
}
func main() {
	
	// Execute the following expression
	// (1+2)*(3+5)
	evaluate("12+35+*")
	// -((7*2) + (6/2) + 4)
	evaluate("72*62/+4+-")
}

Output

12+35+*  =  24
72*62/+4+-  =  -21