Parentheses Check

#include <stdio.h>
char stack [100];
int top;
void pop(){
    top--;
}
void push(char n){
    stack[top++] = n;
}
char peek(){
    return stack[top-1];
}
int main(){
    char str[100];
    int neg = 0;
    printf("\nEnter an expression(! to exit): ");
    gets(string);
    for(int i = 0; string[i] != '\0'; i++){
        switch(string[i])
        {
            case '(':   push('('); break;
            case '[':   push('['); break;
            case '{':   push('{'); break;
            case ')':   if(peek() == '(') pop();
                        else neg = 1;
                        break;
            case ']':   if(peek() == '[') pop();
                        else neg = 1;
                        break;
            case '}':   if(peek() == '{') pop();
                        else neg = 1;
            default:    continue;
        }
    }
    if(top) printf("\nParenthesis Not Valid! Closing Missing.");
    if(neg) printf("\nParenthesis Not Valid! Opening Missing.");
    if(!top && !neg) printf("\nParenthesis Valid!");
}