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

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"

Output: true

Example 2:

Input: "()[]{}"

Output: true

Example 3:

Input: "(]"

Output: false

Example 4:

Input: "([)]"

Output: false

Example 5:

Input: "{[]}"

Output: true

package LeetCode;

import java.util.HashMap;
import java.util.Map;
import java.util.Stack;

public class L20_ValidParentheses {
    public boolean isValid(String s) {
        int len=s.length();
        Stack<Character> stack=new Stack<Character>();
        if(len==0)
            return true;
        if(len%2!=0)
            return false;
        Map<Character,Character> map=new HashMap<Character,Character>();
        map.put(‘(‘,‘)‘);
        map.put(‘[‘,‘]‘);
        map.put(‘{‘,‘}‘);
        map.put(‘)‘,‘ ‘);
        map.put(‘]‘,‘ ‘);
        map.put(‘}‘,‘ ‘);
        //stack.push(s.charAt(0));
        for(int i=0;i<len;i++)
        {
            if (stack.empty())//棧為空
                stack.push(s.charAt(i));
            else if(s.charAt(i)!=map.get(stack.peek()))//或輸入與棧頂不匹配
                stack.push(s.charAt(i));
            else
                stack.pop();
        }
        if(stack.empty())
            return true;
        else
            return false;
    }

    public static void main(String[] args){
        L20_ValidParentheses l20=new L20_ValidParentheses();
        String test="([)]";
        boolean result=l20.isValid(test);
        System.out.println(result);
    }
}

LeetCode 20. Valid Parentheses