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InfixToPostFix.java
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124 lines (102 loc) · 2.62 KB
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//Convert a Infix expression to PostFix
public class InfixToPostFix {
private static Character[] openingDelimiters = {'(','{','['};
private static Character[] closingDelimiters = {')','}',']'};
private static Character[] operators = {'+','-','*','/'};
public String infixToPostfix(String expr) throws Exception{
if(expr == null || expr.isEmpty())
throw new Exception("Null or empty parameter");
StringBuilder result = new StringBuilder();
Stack stack = new Stack();
for(int i = 0; i < expr.length(); i++){
Character c = expr.charAt(i);
//Case 1
if(isOperand(c)){
result.append(c);
}
//Case 2
else if(isClosingDelimiter(c)){
printUntilFindOpeningDelimiter(result, stack, c);
}
//Case 3
else{
printUntilFindLowerPriority(result, stack);
stack.push(c);
}
}
//Case 4
printRemainingOperators(result, stack);
return result.toString();
}
public void printUntilFindOpeningDelimiter(StringBuilder result,
Stack stack, Character c) throws Exception {
Character pop = null;
do{
if(isOpeningDelimiter(stack.peek()))
if(isPair(stack.peek(),c)){
stack.pop();
break;
}
else
throw new Exception("Invalid Expression");
pop = stack.pop();
result.append(pop);
}
while(!isOpeningDelimiter(pop));
}
public void printUntilFindLowerPriority(StringBuilder result,
Stack stack) {
Character pop = null;
if(!stack.isEmpty())
do{
if(isOpeningDelimiter(stack.peek())) break;
pop = stack.pop();
result.append(pop);
}
while(pop!= null && !stack.isEmpty() && !isOperand(pop) );
}
public void printRemainingOperators(StringBuilder result,
Stack stack) {
if(!stack.isEmpty())
do{
Character pop = stack.pop();
result.append(pop);
}while(!stack.isEmpty());
}
private boolean isOperand(Character c) {
return !isDelimiter(c) && !isOperator(c);
}
private boolean isDelimiter(Character c){
return isOpeningDelimiter(c) || isClosingDelimiter(c);
}
private boolean isPair(Character open, Character close) {
switch(open){
case '(' :
return close.equals(')');
case '{' :
return close.equals('}');
case '[' :
return close.equals(']');
}
return false;
}
private boolean isOpeningDelimiter(Character c) {
for(Character d : openingDelimiters)
if(c.equals(d))
return true;
return false;
}
private boolean isClosingDelimiter(Character c) {
;
for(Character d : closingDelimiters)
if(c.equals(d))
return true;
return false;
}
private boolean isOperator(Character c){
for(Character d : operators)
if(c.equals(d))
return true;
return false;
}
}