UVa 10945 - Mother Bear * (palindrome)

Link:https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=21&page=show_problem&problem=1886
Sol:

#include <iostream>
	#include <string>
	using namespace std;
	bool IsInvalid(const char& letter)
	{
	return (letter == '.' || letter == ',' || letter == '!' || letter == '?' || letter == ' ');
	}
	bool IsPalindrome(const string& word)
	{
	int start = 0;
	int end = word.size() -1;
	while (start < end && IsInvalid(word[start]))
	++start;
	while (end > start && IsInvalid(word[end]))
	--end;
	while (start < end)
	{
	if (tolower(word[start]) != tolower(word[end]))
	return false;
	++start;
	--end;
	while (start < end && IsInvalid(word[start]))
	++start;
	while (end > start && IsInvalid(word[end]))
	--end;
	}
	}
	int main()
	{
	string word;
	while (getline(cin, word), word != "DONE")
	{
	if (IsPalindrome(word))
	cout << "You won't be eaten!\n";
	else
	cout << "Uh oh..\n";
	}
	}