有效的括号 Java

Question

I am trying to solve the Valid Parentheses problem in Java (described here among other places: Valid Parentheses )

My code is as follows thus far:

class Solution {
    public boolean isValid(String s) {

        if(s.charAt(0) ="(") {
            if(s.charAt(s.length-1) != ")"){
                system.out.println("false");
            }
        }

        if(s.charAt(0) ="[") {
            if(s.charAt(s.length-1) != "]"){
               system.out.println("false");
            }
        }

        if(s.charAt(0) ="{") {
            if(s.charAt(s.length-1) != "}"){
                system.out.println("false");
            }
        }       
    }
}

Currently I am getting the following error:

Line 4: error: unexpected type

   if(s.charAt(0) ="("){

Can anyone advise on what the issue is? I can't at the moment figure it out but feel like it's something simple I'm overlooking.

Answer 1

Two immediate issues:

1. Character values are single-quoted in Java, eg char c = 'c';
2. Primitive comparison is done with == . The single equals sign = is used for assignment.

So that line should be:

if(s.charAt(0) == '(') { /* ... */ }

Note that this only fixes the most immediate error you're experiencing. After fixing that, there are some more, for example system.out.println() should be System.out.println() .

And, after fixing that, I think you're still a way off from solving the actual assignment. Just keep at it & good luck!

Answer 2

There are some issues in your answer. Robby Cornelissen points you out some issues in his answer . And I also saw 2 errors in your code.

1. s.length should be s.length() .
2. You don't have a return statement in the isValid method.

Issue 1

In the following statement, you are missing the brackets () after the length. Length is a method in Java that is used to get the length of a Java String that you know already.

if(s.charAt(s.length - 1) != ')')

So your above statement should be as follows.

if(s.charAt(s.length() - 1) != ')')

Issue 2

In your isValid method it's returning a boolean value. The return type of that method is boolean . But you are not returning any boolean value inside your method. So you have to add a return statement. It may be,

return true; or return false; or any boolean variable declared inside isValid method.

Answer 3

it is best to use stack for valid parentheses in Java

class Solution {
public boolean isValid(String s) {
   Stack stack = new Stack();
    for(int i=0; i<s.length; i++){
     if(s[i] =='('){
       stack.push(')')
    }
    else if(s[i] == '['){
     stack.push(']')
    }
    else if(s[i] =='{'){
     stack.push('}')
    }
   else if(s[i] !== stack.pop()){
    return false;
    }    
          
     }
   return true;
  }
}