ValidParentheses.java

/*
Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.
*/


public class Solution {
    public boolean isValid(String s) {
        if (s==null || s.length()==0){
            return true;
        }
        
        Stack<Character> checker=new Stack<Character>();
        
        int i=0;
        while (i<s.length()){
            
            if (s.charAt(i)=='(' || s.charAt(i)=='[' || s.charAt(i)=='{'){
                checker.push(s.charAt(i));
                i++;
            }
            else if (s.charAt(i)==')' || s.charAt(i)==']' || s.charAt(i)=='}'){ 
                if (checker.isEmpty()){
                    return false;
                }
                else{
                    char temp=checker.peek();
                    if (isPair(temp, s.charAt(i))){
                        checker.pop();
                        i++;
                    }
                    else{
                        return false;
                    }
                }
            }
            else{
                return false;
            }
            
        }
        
        if (checker.isEmpty() ){
            
            return true;
        }
        
        return false;
    }
    // check is given two chars are pair
    private boolean isPair(char c1, char c2){
        String validChars="()[]{}";
        
        
        int indexC1=validChars.indexOf(c1);
        int indexC2=validChars.indexOf(c2);
        
        if (indexC1<0||indexC2<0){
            return false;
        }
        
        if (indexC1+1==indexC2){
            return true;
        }
         
        return false;
    }
}