package com.coding.basic.stack.expr;
import java.util.List;
import java.util.Stack;
public class PostfixExpr {
String expr = null;
public PostfixExpr(String expr) {
this.expr = expr;
}
public float evaluate() {
Stack<Float> numStack = new Stack<>();
TokenParser parser = new TokenParser();
List<Token> tokens = parser.parse(expr);
for(Token token : tokens) {
if(token.isNumber()){
numStack.push(new Float(token.getIntValue()));
}else{
String op = token.value;
float num2 = numStack.pop();
float num1 = numStack.pop();
float result = calculate(op, num1, num2);
numStack.push(result);
}
}
return numStack.pop();
}
private Float calculate(String op, Float f1, Float f2){
if(op.equals("+")){
return f1+f2;
}
if(op.equals("-")){
return f1-f2;
}
if(op.equals("*")){
return f1*f2;
}
if(op.equals("/")){
return f1/f2;
}
throw new RuntimeException(op + " is not supported");
}
private void reverse(Stack s){
Stack tmp = new Stack();
int len = s.size();
for(int i=0;i<len;i++){
Object top = s.pop();
while(s.size() > i) {
tmp.push(s.pop());
}
s.push(top);
while(tmp.size() > 0){
s.push(tmp.pop());
}
}
}
}