package com.coding.basic.stack.expression;
import java.util.List;
import java.util.Stack;
/*
* 前序表达式求值
* 与后序表达式求值相反,相当于将后序表达式逆置,然后按照后续表达式的规则求值
*
*/
public class PrefixExpr {
String expr = null;
public PrefixExpr(String expr) {
this.expr = expr;
}
public float evaluate() {
TokenParser parser = new TokenParser();
List<Token> tokens = parser.parse(expr);
Stack<Token> opStack = new Stack<>();
Stack<Float> numStack = new Stack<>();
for(int i = tokens.size()-1; i>=0; i--){
Token token = tokens.get(i);
if(token.isOperator()){
if(opStack.isEmpty()){
opStack.push(token);
} else {
float f1 = numStack.pop();
float f2 = numStack.pop();
numStack.push(calculate(opStack.pop().value,f1,f2));
opStack.push(token);
}
}
if(token.isNumber()){
if(numStack.isEmpty()){
numStack.push((float) token.getIntValue());
} else {
if (opStack.isEmpty()){
numStack.push((float) token.getIntValue());
} else {
float f1 = numStack.pop();
float f2 = numStack.pop();
numStack.push(calculate(opStack.pop().value,f1,f2));
numStack.push((float) token.getIntValue());
}
}
}
}
while(!opStack.isEmpty()){
float f1 = numStack.pop();
float f2 = numStack.pop();
numStack.push(calculate(opStack.pop().value,f1,f2));
}
return numStack.pop();
}
private float calculate(String value, float f1, float f2) {
if(value.equals("+")){
return f1 + f2;
}
if(value.equals("-")){
return f1 - f2;
}
if(value.equals("*")){
return f1 * f2;
}
if(value.equals("/")){
return f1 / f2;
}
return -1;
}
public static void main(String[] args){
PrefixExpr expr = new PrefixExpr("-++6/*2 9 3 * 4 2 8");
System.out.println(expr.evaluate());
}
}