20. Valid Parentheses检验括号字符串的有效性

Given a string s 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.

 

Example 1:

Input: s = "()"
Output: true

Example 2:

Input: s = "()[]{}"
Output: true

Example 3:

Input: s = "(]"
Output: false

Example 4:

Input: s = "([)]"
Output: false

Example 5:

Input: s = "{[]}"
Output: true

这种带技巧的题，真的忘了

思路是：先全部push进去，再pop

stack.pop() == null会出异常，所以应该是stack.isEmpty()

最后检查一下stack是否为空

class Solution {
    public boolean isValid(String s) {
        //cc
        if (s == null || s == "")
            return true;
        
        //新建一个stack，遍历s中的每一个元素
        Stack<Character> stack = new Stack<Character>();
        
        //返回
        for (char c : s.toCharArray()) {
            if (c == '(') {
                stack.push(')');
            } else if (c == '{') {
                stack.push('}');
            } else if (c == '[') {
                stack.push(']');
            } else {
                if (stack.pop() == null || stack.pop() != c)
                    return false;
            }
        }
        
        return stack.isEmpty();
    }
}