Valid Parentheses- Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if: Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order. Note that an empty string is also considered valid.

validParentheses.java

     class Solution {
    public boolean isValid(String s) {
        HashMap<Character,Character> maps=new HashMap<Character,Character>();
        maps.put(')','(');
        maps.put(']','[');
        maps.put('}','{');
        Stack<Character> stack=new Stack<Character>();
        for(int i=0;i<s.length();i++){
            char c=s.charAt(i);
            if(maps.containsKey(c)){
                if(stack.empty()||stack.pop()!=maps.get(c))return false;
            }
            else
                stack.push(c);
        }
        return stack.empty();
    }
}