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.

一开始把题目想的太简单，不知道（{}）这种包含括号的存在。

其实这道题很简单，用栈就可以解决。把遇到的字符压入栈，但新的字符与栈顶元素配对就弹出栈顶，不匹配就把新字符压入栈，这样最后看栈是否为空来判断input string is valid。

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public class Solution {
     public boolean isValid(String s) {
        HashMap<Character,Character> map = new HashMap<Character,Character>();
        boolean result = true;
        if(s.length() == 0)return true;
        map.put('(', ')');
        map.put('[', ']');
        map.put('{', '}');
        map.put(']', '#');
        map.put(')', '#');
        map.put('}', '#');
        char[] ch = s.toCharArray();
        Stack<Character> stack = new Stack<Character>();
        stack.push(ch[0]);
        int i = 1;
        while(i<ch.length)
        {
            char key = ch[i];
            if(!stack.empty()&&map.get(stack.peek())==key)
            {
                stack.pop();
            }
            else
            {
                stack.push(ch[i]);
            }
            i++;
           
        }
        result = stack.empty();
        return result;
    }
}