20.c

bool isValid(char * s){
    int i, k = 0, len = strlen(s);
    char *store = calloc(len, sizeof(char));

    for( i = 0; s[i] != '\0'; i++) {
        switch(s[i]) {
            case '(':
            case '{':
            case '[':
                store[k++] = s[i];
                break;
            case ')':
                if(k < 1 || store[--k] != '(')
                    goto out;
                break;
            case '}':
                if(k < 1 || store[--k] != '{')
                    goto out;
                break;
            case ']':
                if(k < 1 || store[--k] != '[')
                    goto out;
                break;
        }
    }
out:
    free(store);
    return s[i] == '\0' && k == 0;
}