https://leetcode-cn.com/problems/valid-parentheses/

    1.  public boolean isValid4(String s) {
    2.      if (s.length()==0 || s.length()==1) {
    3.          return false;
    4.      }
    5.      HashMap<Character, Character> newmap = new HashMap<>();
    6.      newmap.put('(', ')');
    7.      newmap.put('[', ']');
    8.      newmap.put('{', '}');
    9.      Stack<Character> stack = new Stack<>();
    10.      for (int i = 0; i < s.length(); i++) {
    11.          char c = s.charAt(i);
    12.          if (c == '(' || c == '[' || c == '{' ) {
    13.              stack.push(c);
    14.          }else {
    15.              if (stack.isEmpty()) return false;
    16.              char left = stack.pop();
    17.              if (c != newmap.get(left)) {
    18.                  return false;
    19.              }
    20.          }
    21.      }
    23.      return stack.isEmpty();
    24.  }

    20年b站的面试题，第二遍做