An input string is valid if:

Open brackets must be closed by the same type of brackets.
Open brackets must be closed in the correct order.
Note that an empty string is also considered valid.

bool isValid(char* s) {

int k=0,i=0;
int p;
p=strlen(s);
int MAXSIZE=10000;//輸入較大時，可能空間不足

char st;
st=(char)malloc(MAXSIZE*sizeof(char));

if(s==NULL)
return true;
if(p%2==1)
return false;
while(s[k]){
if((s[k]==‘{‘)||(s[k]==‘[‘)||(s[k]==‘(‘)){
st[i]=s[k];
++k;
++i;}
else{
switch(s[k]){
case ‘}‘:{
if(st[i-1]==‘{‘)//註意比較的數組以及序號

      break;}
       case ‘]‘:{
              if(st[i-1]==‘[‘)
                  --i;
      else 
          return false;
            ++k;
      break;}
       case ‘)‘:{
              if(st[i-1]==‘(‘)
                  --i;
      else 
          return false;

        ++k;    
      break;}
  }
 }
 

}
if(i==0)
return true;
else
return false;
}

棧的運用（ Valid Parentheses）