02. Brackets

#include <iostream>
#include <string>

using namespace std;

int main() {
    string expression,
        openBrackets = "([{",
        closedBrackets = ")]}";
    cin >> expression;

    if (expression.length() % 2 == 0) {
        while (expression.length() > 0 && openBrackets.length() > 0) {
            for (size_t i = 1; i < expression.length(); i++) {
                if (expression[i - 1] == openBrackets[0] && expression[i] == closedBrackets[0]) {
                    expression.erase(i - 1, 2);
                    i = 0;
                }
            }
            openBrackets.erase(0, 1);
            closedBrackets.erase(0, 1);
        }
    }

    cout << (expression.length() > 0 ? "invalid" : "valid") << endl;
    return 0;
}