括号对称匹配算法

class Solution {

    private HashMap<Character,Character> mappings;

    public Solution() {

        //构建一个存放对应括号的map

        this.mappings = new HashMap<Character,Character>();

        this.mappings.put(')','(');

        this.mappings.put('}','{');

        this.mappings.put(']','[');

    }

    public boolean isValid(String s) {

        //初始化栈

        Stack<Character> stack = new Stack<Character>();

        for (int i=0; i<s.length(); i++) {

            char c = s.charAt(i);

            //如果出现右括号，匹配栈顶

            if (this.mappings.containsKey(c)) {

                char topElement = stack.empty() ?'#':stack.pop();

                if(topElement != this.mappings.get(c)) {

                    return false;

                }

            }else {

                stack.push(c);

            }

        }

        return stack.isEmpty();

    }

}