후위 표현식

ProgrammingAlgorithm
Algorithm, 후위 표현식

중위 표현식을 후위 표현식으로 변경하라.

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ex) 5 + (4 - 2) - 3 * 2  ===  5 4 2 - + 3 2 * -

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import java.util.*;
public class Postfix{
  public static void main(String[] args){
    String expression = "5+(4-2)-3*2";
    Stack stack = new Stack();
    StringBuilder sb = new StringBuilder();
    for(int i = 0; i < expression.length(); i++){
      char c = expression.charAt(i);
      switch(c){
        case '0':
        case '1':
        case '2':
        case '3':
        case '4':
        case '5':
        case '6':
        case '7':
        case '8':
        case '9':
        sb.append(c);
        break;
        case '+':
        case '-':
        case '*':
        case '/':
        if(!stack.empty() && (stack.peek() != '(')){
          if(order(stack.peek()) <= order(c)){
            sb.append(stack.pop());
          }
        }
        stack.push(c);
        break;
        case '(':
        stack.push(c);
        break;
        case ')':
        char op;
        while((op = stack.pop()) != '('){
          sb.append(op);
        }
        break;
        default:
        System.out.println("Error");
        return;
      }
    }
    while(!stack.empty()){
      sb.append(stack.pop());
    }
    System.out.println(sb.toString());
  }
  public static int order(char op){
    switch(op){
      case '*': case '/': return 1;
      case '+': case '-': return 2;
      default: return 0; }
    }
  }
}
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