當 vector 為 empty 時
left.pop_back 會造成錯誤
left.back 可能也會造成錯誤(我沒仔細測,不確定)
解決的方法是
在對 vector 做「讀/取動作」之前
先檢查是否是 empty
如果是 empty 表示符號不成對-->return false
如果不是 empty 才繼續往下做
簡單修補程式如下

#include <iostream>
#include <vector>

using namespace std;

bool isValid(string s) {
    vector<char> left;//store (, [, {
    for( auto c : s ){
        switch(c){
                case('('):
                case('['):
                case('{'):
                //store in left vector with order
                    left.push_back(c);
                
                    break;
                //for the other cases
                case(')'):
                    if (left.empty()) { return false; }
                    if(left.back() == '('){
                        left.pop_back();
                //the last is matched, pop out
                    }
                    else if(left.back() != '(' || left.empty() ){
                        return false;
                        //if not match the last letter or no any letter
                    }
                     break;  //end case 
                case(']'):
                    if (left.empty()) { return false; }
                    if(left.back() == '['){
                        left.pop_back();
                    }
                    else if(left.back() != '[' || left.empty() ){
                        return false;
                    }
                     break; 
                case('}'):
                    if (left.empty()) { return false; }
                    if(left.back() == '{'){
                        left.pop_back();
                    }
                    else if(left.back() != '{' || left.empty() ){
                        return false;
                    }
                     break; 
            default: //the string start and end with ' '
                break;
        }
    }
    if( !left.empty() )return false;//no any letter not match
    return true;
}

int main()
{
   string s = "([])";
   cout << s << " is " << isValid(s) << endl; 
   s = "([)]";
   cout << s << " is " << isValid(s) << endl; 
   s = "}";
   cout << s << " is " << isValid(s) << endl; 
   s = "[[[]]]]";
   cout << s << " is " << isValid(s) << endl; 
   
   return 0;
}