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InfixToPostfix.java

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package DataStructures.Stacks;

import java.util.Stack;

public class InfixToPostfix {

public static void main(String[] args) throws Exception {

assert "32+".equals(infix2PostFix("3+2"));

assert "123++".equals(infix2PostFix("1+(2+3)"));

assert "34+5*6-".equals(infix2PostFix("(3+4)*5-6"));

}

public static String infix2PostFix(String infixExpression) throws Exception {

if (!BalancedBrackets.isBalanced(infixExpression)) {

throw new Exception("invalid expression");

}

StringBuilder output = new StringBuilder();

Stack<Character> stack = new Stack<>();

for (char element : infixExpression.toCharArray()) {

if (Character.isLetterOrDigit(element)) {

output.append(element);

} else if (element == '(') {

stack.push(element);

} else if (element == ')') {

while (!stack.isEmpty() && stack.peek() != '(') {

output.append(stack.pop());

}

stack.pop();

} else {

while (!stack.isEmpty() && precedence(element) <= precedence(stack.peek())) {

output.append(stack.pop());

}

stack.push(element);

}

}

while (!stack.isEmpty()) {

output.append(stack.pop());

}

return output.toString();

}

private static int precedence(char operator) {

switch (operator) {

case '+':

case '-':

return 0;

case '*':

case '/':

return 1;

case '^':

return 2;

default:

return -1;

}

}

}