UVA - 673 Parentheses Balance

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<ctype.h>
using namespace std;
char str[150];
char stack[150];
int main() {
int n;
scanf("%d", &n);
getchar();
while (n--) {
memset (stack, 0, sizeof(stack));
gets(str);
int tag = 0;
int len = strlen(str);
for (int i = 0; i < len; i++) {
if (str[i] == '(' || str[i] == '[') {
stack[tag] = str[i];
tag++;
}
if (str[i] == ')') {
if (stack[tag-1] == '(') {
stack[tag-1] = '\0';
tag--;
}
else {
stack[tag] = str[i];
tag++;
}
}
if (str[i] == ']') {
if (stack[tag-1] == '[') {
stack[tag-1] = '\0';
tag--;
}
else {
stack[tag] = str[i];
tag++;
}
}
}
if (stack[0] == '\0')
printf("Yes\n");
else
printf("No\n");
}
return 0;
}