convert expression To Postfix - Java Data Structure

Java examples for Data Structure:Inverse Poland Expression

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Description

convert expression To Postfix

Demo Code

import java.util.HashMap;
import java.util.Map;
import java.util.Stack;

public class Main {

  private static String convertToPostfix(String expr) {
    Map<Character, Integer> opPrecedence = new HashMap<>();
    opPrecedence.put('+', 0);
    opPrecedence.put('-', 0);
    opPrecedence.put('*', 1);
    opPrecedence.put('/', 1);

    Stack<Character> stack = new Stack<>();
    String output = "";

    int i = 0;//from  w w  w .  j  a v a2  s .c  o m
    while (i < expr.length()) {
      char cur = expr.charAt(i);

      if (cur == ' ') {
        ++i;
        continue;
      }

      if (Character.isDigit(cur)) {
        String numStr = "";

        while (i < expr.length() && Character.isDigit(expr.charAt(i))) {
          numStr += expr.charAt(i);
          ++i;
        }

        output += numStr;
        output += ' ';

        continue;
      }

      if (cur == '(') {
        stack.push(cur);

        ++i;

        continue;
      }

      if (cur == ')') {
        while (!stack.empty() && stack.peek() != '(') {
          output += stack.pop();
          output += ' ';
        }

        if (stack.empty()) {
          throw new IllegalArgumentException("Invalid expression");
        }

        stack.pop();

        ++i;

        continue;
      }

      if (cur == '+' || cur == '-' || cur == '*' || cur == '/') {
        while (!stack.empty() && opPrecedence.containsKey(stack.peek())
            && opPrecedence.get(stack.peek()) >= opPrecedence.get(cur)) {
          output += stack.pop();
          output += ' ';
        }

        stack.push(cur);

        ++i;

        continue;
      }

      throw new IllegalArgumentException("Invalid expression");
    }

    while (!stack.empty()) {
      if (stack.peek() == '(') {
        throw new IllegalArgumentException("Invalid expression");
      }

      output += stack.pop();
      output += ' ';
    }

    return output;
  }

  public static void main(String[] args) {
    String expr = "(1 - 5) * 3 + 12 * 3 / 4 * (1 - 3) / 3";
    System.out.println(convertToPostfix(expr));

  }
}

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