1class Solution {
2  public:
3    bool isValid(string s) {
4      vector<char> stack;
5
6      for(char c : s) {
7        if(c==40 || c==91 || c==123) {
8          stack.push_back(c);
9        } else {
10          if(stack.size() == 0) return false;
11
12          char p = stack.back();
13          if((p==40 && c==41) || (p==91 && c==93) || (p==123 && c==125)) stack.pop_back();
14          else return false;
15        }
16      }
17
18      return stack.size() == 0;
19    }
20};

1# @param {String} s
2# @return {Boolean}
3def is_valid(s)
4  return false if s[0] == ')' or s[0] == '}' or s[0] == ']'
5  return false if (s.size)&1 == 1
6
7  stack = []
8  size = s.size
9
10  for i in 0...size do
11    if s[i] == '(' or s[i] == '[' or s[i] == '{'
12      stack.push(s[i])
13    elsif s[i] == ')'
14      top = stack.pop
15      return false if top != '('
16    elsif s[i] == ']'
17      top = stack.pop
18      return false if top != '['
19    elsif s[i] == '}'
20      top = stack.pop
21      return false if top != '{'
22    end
23  end 
24  return stack.empty?
25end