I should be getting "invalid" as output but I'm getting "valid" as output, don't know why the output is wrong

Question

I wrote a program to check whether the given mathematical expression is correct or not but in the three test cases I had taken from web two of them getting correct output but one of the test case getting wrong output, but I don't know why..?

Test cases that I got right output are "(a+b)(a*b)" and "((a+b)". The test case that I'm getting wrong output is "(ab)(ab+)"

The code I tried is

 public static void main(String[] args) {
        // TODO code application logic here
      Scanner sc=new Scanner(System.in);
      String str=sc.next();
      int l=str.length();
      int a1=0,t=0,k=0;
      for(int i=0;i<l;i++){
          if(str.charAt(i)=='('){
              a1++;
          }else if(str.charAt(i)==')'){
              a1--;
          }
          if(str.charAt(i)=='+'||str.charAt(i)=='-'||str.charAt(i)=='*'||str.charAt(i)=='/'){
              k++;
              if(str.charAt(--i)>='a'&&str.charAt(--i)<='z'&&str.charAt(++i)>='a'&&str.charAt(++i)<='z'){
                  t++;
              }
          }
      }
      if(a1==0&&t==k){
          System.out.println("valid");
      }else{
          System.out.println("invalid");
      }
    }

The output expected for test case 3 is invalid but it gives valid as output.

Answer 1

You should not write .charAt(--i) or .charAt(++i) . Change it to i-1 and i+1 . What ++i and --i do is change the value of i when it is being checked.