gistfile1.txt

/*
Given a string s 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.
Every close bracket has a corresponding open bracket of the same type.
 

Example 1:
Input: s = "()"
Output: true

Example 2:
Input: s = "()[]{}"
Output: true

Example 3:
Input: s = "()[{}"
Output: false


Example 4(nested):
Input: s = "[(){}]"
Output: true


Example 5:
Input: s = "(({)})"
Output: false
*/
void main() {
  test();
}

bool isValid(String input) {
  final List<int> stack = [];
  for (int i = 0; i < input.length; i++) {
    int aChar = input.codeUnitAt(i);
    if (aChar == '('.codeUnitAt(0) ||
        aChar == '{'.codeUnitAt(0) ||
        aChar == '['.codeUnitAt(0)) {
      stack.add(aChar);
    } else {
      if (stack.isEmpty) {
        return false;
      }

      int lastCh = stack.removeLast();
      if (aChar == ')'.codeUnitAt(0) && lastCh != '('.codeUnitAt(0)) {
        return false;
      }

      if (aChar == '}'.codeUnitAt(0) && lastCh != '{'.codeUnitAt(0)) {
        return false;
      }
      if (aChar == ']'.codeUnitAt(0) && lastCh != '['.codeUnitAt(0)) {
        return false;
      }
    }
  }
  return stack.isEmpty;
}

void test1(String input, bool expected) {
  bool actual = isValid(input);
  String valid = (expected == actual) ? "\u2713" : " x ";
  print("expected=$expected, actual=$actual $valid");
}

void test() {
  test1("[]", true);
  test1("[{}]", true);
  test1("[{}]({})", true);
  test1("[()]{}", true);
  test1("[{(})]", false);
  test1("[()]}", false);
  test1("{[()]", false);
}