Leetcode Valid Parentheses

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.

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[code]

public class Solution {


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