20. Valid Parentheses [effective brackets]

Description
given only includes a '(', ')' string, '{', '}', '[', ']', and determine whether the string is valid.

Valid string must meet:

Left bracket must be closed by a closing parenthesis of the same type.
Left parenthesis must be closed in the correct order.
Note the empty string can be considered a valid string

Examples of

ideas

• Use the stack, each of the brackets turn into the stack, when the stack of elements coming into the brackets and top of the stack is the first one pairs [go into a left parenthesis, followed by the closing parenthesis comparison], then the top element pop-up, otherwise, the brackets into the stack.
  answer
• python

python有list有pop()函数，使用list代替stack
class Solution:
    def isValid(self, s: str) -> bool:
            
        stack=[]
        d={'(':')','[':']','{':'}'}
        
        for c in s:#字符串中的字符不能直接通过下标进行修改 
            if len(stack)>0 and c==d.get(stack[-1],'?'):
            #c为),},或]，top中的应为左括号 ([)()])不是有效的括号
                stack.pop()
            else:
                stack.append(c)
              
        return len(stack)==0

• c++

class Solution {
public:
    bool isValid(string s) {
        stack<char> st;
        unordered_map<char,char> dict={{'(',')'},{'[',']'},{'{','}'}};
        for (char c: s)
        {   
            if (!st.empty() && (dict[st.top()]==c))
                st.pop();
            else
                st.push(c);
        }
        return st.empty();
        
        
    }
};