week8 stack

//1.why: Go back, check closest first
//2.how: 
// scanner: case_1: stack.offerLast()
//sanner: case_2.1: stack.peekLast()-->stack.pollLast()
//scanner: case_3: stack.pollLast()
//scanner: case_2.2: stack.peekLast()-->stack.pollLast() 两者相互循环

public int evallnFix(char[] tokens){
   Deque<Integer> valStack = new LinkedList<Integer>(); //stack to store numbers
   Deque<Integer> opStack = new LinkedList<Character>(); // stack to store operators
   
   for(int i = 0; i < tokens.length; i++){
     if(token[i] == '('){ Case 1:'('
       opStack.offerLast(toekns[i]);
       }else if (tokens[i] == ')'){//Case 2: ')'
          while (opStack.peekLast()!='('){
            valStack.offerLast(cal(opStack.pollLast(), valStack.pollLast(), valStack.pollLast())); 
            }
           opStack.pollLast();//poll '('
           }else if (tokens[i] == '+' || tokens[i] == '-'|| tokens[i] == '*'|| tokens[i] == '/'){ // Case 3: Operators other than parentheses
             while(!opStack.isEmpty() && !isHigherThan(t=okens[i], opStack.peekLast())){
                valStack.offerLast(cal(opStack.pollLast(), valStack.pollLast(), valStack.pollLast()));
                }
                opStack.offerLast(tokens[i]);// offer current operator
                } else{ // Case 4: number
                   valStack.offerLast(Integer.parseInt(tokens[i]));
                }
              }
            //Calculate all numbers rest in stack
            while(!opStack.isEmpty()){
              valStack.offerLast(cal(opStack.pollLast(), valStack.pollLast(), valStack.pollLast()));
              }
            return valStack.pollLast();
          }