PTA上的一道括号匹配检测题目，有两个检测点过不去，是有哪些没考虑到的问题吗？(附上题目和代码)

#include<cstdio>
#include<cstdlib>
#include<stack>
#include<cstring>
using namespace std;
int main(){
    char str[110];
    stack<char> stk;
    fgets(str, 110, stdin);
    str[strcspn(str, "\n")] = 0;
    int n = 0;
    int i = 0;
    while(str[i]!='\0'){
        if(str[i]=='(' || str[i]=='[' || str[i]=='{'){                 //遇见左括号进栈     
            stk.push(str[i]);
        }
        else if(str[i]==')' || str[i]==']' || str[i]=='}'){            //遇见右括号匹配
            if(stk.empty()){                                          //栈为空无法匹配的情况
                i++;
                continue;
            }
            else{                                                     //栈不为空，左右对应可匹配的情况
                if(str[i]==')' && stk.top()=='('){
                    stk.pop();
                    n++;
                }
                else if(str[i]==']' && stk.top()=='['){
                    stk.pop();
                    n++;
                }
                else if(str[i]=='}' && stk.top()=='{'){
                    stk.pop();
                    n++;
                }
                else{                                                //栈不为空，左右不对应不可匹配的情况
                    i++;
                    continue;
                }
                    
            }
        }
        i++;
    }
    printf("%d\n",n);
}