以下是判断一个字符串是否为一个回文，请问这段代码是如何忽略标点符号的？
#include
#include
#include
#define YES 1
#define NO 0
/* stackNode structure definition /
struct stackNode {
char data; / node data /
struct stackNode *nextPtr; / pointer to next node /
}; / end struct stackNode */
typedef struct stackNode STACKNODE;
typedef STACKNODE *STACKNODEPTR;
/* function prototypes */
void push( STACKNODEPTR *topPtr, char info );
char pop( STACKNODEPTR *topPtr );
int isEmpty( STACKNODEPTR topPtr );
int main( void )
{
STACKNODEPTR stackPtr = NULL; /* points to the stack top /
char c; / current character from text /
char line[ 50 ]; / text from user /
char condensedLine[ 50 ]; / text with only letters /
int i = 0; / length of condensed line /
int j = 0; / length of line /
int palindrome = YES; / result of palindrome test */
printf( "Enter a line of text:\n" );
/* read each letter with getchar and add to line */
while ( ( c = getchar() ) != '\n' ) {
line[ j++ ] = c;
/* remove all spaces and punctuation */
if ( isalpha( c ) ) {
condensedLine[ i++ ] = tolower( c );
push( &stackPtr, tolower( c ) );
} /* end if */
} /* end while */
line[ j ] = '\0';
/* loop through condensedLine */
for ( j = 0; j < i; j++ ) {
/* if condensedLine does not equal stack */
if ( condensedLine[ j ] != pop( &stackPtr ) ) {
palindrome = NO;
break; /* exit loop */
} /* end if */
} /* end for */
/* if text is a palindrome /
if ( palindrome ) {
printf( "\"%s\" is a palindrome\n", line );
} / end if /
else {
printf( "\"%s\" is not a palindrome\n", line );
} / end else */
return 0; /* indicate successful termination */
} /* end main */
/* Insert a node at the stack top /
void push( STACKNODEPTR *topPtr, char info )
{
STACKNODEPTR newPtr; / temporary node pointer */
/* dynamically allocate memory */
newPtr = malloc( sizeof( STACKNODE ) );
/* if memory was allocated, insert node at top of stack /
if ( newPtr ) {
newPtr->data = info;
newPtr->nextPtr = *topPtr;
*topPtr = newPtr;
} / end if /
else {
printf( "%d not inserted. No memory available.\n", info );
} / end else */
} /* end function push */
/* Remove a node from the stack top /
char pop( STACKNODEPTR *topPtr )
{
STACKNODEPTR tempPtr; / temporary node pointer /
int popValue; / value of popped node */
tempPtr = topPtr;
popValue = ( *topPtr )->data;
*topPtr = ( *topPtr )->nextPtr; / reset topPtr /
free( tempPtr ); / free memory */
return popValue; /* return value of popped node */
} /* end function pop */
/* Is the stack empty? /
int isEmpty( STACKNODEPTR topPtr )
{
return !topPtr; / return NULL if stack is empty */
} /* end function isEmpty */
