LeetCode : 20. Valid Parentheses 验证括号

试题
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.

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
代码
关键点：如果相邻两个char能够组合成一对括号，那么他们肯定是一对。这样的话我们可以优先把这些消掉。

class Solution {
    public boolean isValid(String s) {
        if(s==null || s.length()==0) return true;
        Stack<Character> sta = new Stack<>();
        for(int i=0; i<s.length(); i++){
            char cur = s.charAt(i);
            if(sta.isEmpty()){
                sta.push(cur);
                continue;
            }
            if( (cur==')' && sta.peek()=='(') || (cur==']' && sta.peek()=='[') || (cur=='}' && sta.peek()=='{') ){
                sta.pop();
            }else{
                sta.push(cur);
            }
        }
        return sta.isEmpty();
    }
}