Brackets.java

package Brackets;

// you can also use imports, for example:
import java.util.*;

class Solution {
    public int solution(String S) {

        // main idea: use "Stack" (push and pop)
        
        //special case
        if(S.length() == 0)
            return 1;
        
        // new Stack<Character>()
        Stack<Character> stack = new Stack<>();
        
        // scan the string (just one pass)
        for(int i=0; i< S.length(); i++){    
            // note: push "its pair"
            if( S.charAt(i) == '(' ){
                stack.push(')');
            }
            else if( S.charAt(i) == '[' ){
                stack.push(']');
            }
            else if( S.charAt(i) == '{' ){
                stack.push('}');
            }
            // pop and check
            else if( S.charAt(i) == ')' || S.charAt(i) == ']' || S.charAt(i) == '}'){
                // important: check if the stack is empty or not (be careful)
                if(stack.isEmpty() == true){
                    return 0;
                }
                else{
                    char temp = stack.pop(); // check if the stack is empty before pop!!!
                    if(temp != S.charAt(i)){ // not a pair
                        return 0;
                    }
                }
            }
        }
        // note: check if the stack is empty or not (be careful)
        if( !stack.isEmpty() ){
            return 0;
        }
        else{
            return 1;
        }
    }
}