infixToPostfix.java

import java.util.Stack;

public class infixToPostfix {

    public static void main(String[] args) {


        String s = "A+B*C+D";

        System.out.println(infixToPostfix(s));

    }

    static String infixToPostfix(String s){

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

        String postfix = "";

        char[] ch = s.toCharArray();

        for(char c : ch){

            if(c != '+' && c!= '-' && c != '*' && c != '/' && c != '(' && c != ')'){

                postfix = postfix + c;
            }
            else if(c == '('){
                st.push(c);
            }
            else if( c == ')'){
                while (!st.isEmpty()){
                    char t = st.pop();
                    if(t != '('){
                        postfix = postfix + t;
                    }
                    else{
                        break;
                    }
                }
            }
            else if(c == '+' || c=='-' || c == '*' || c == '/'){

                if(st.isEmpty()){
                    st.push(c);
                }
                else {
                    while(!st.isEmpty()){
                        char t = st.pop();
                        if(t == '('){
                            st.push(t);
                            break;
                        }
                        else if(t == '+' || t =='-' || t == '*' || t == '/'){
                            if(getPriority(t)<getPriority(c)){
                                st.push(t);
                                break;
                            }
                            else{
                                postfix = postfix + t;
                            }
                        }
                    }

                    st.push(c);

                }
            }
        }

        while(!st.isEmpty()){
            postfix = postfix + st.pop();
        }

        return postfix;

    }

    static int getPriority(char c){

        if(c == '+' || c == '-'){
            return 1;
        }
        else{
            return 2;
        }
    }


}