Giving Correct Answer

    bool is_palindrome(string s,int start,int end){
    while(start<=end){
        if(s[start  ]!=s[end--]){
            return false;
        }
    }return true;
} 

Giving Wrong Answer

bool is_palindrome(string s,int start,int end){
    while(start<=end){
        if(s[start  ]==s[end--]){
            return true;
        }
    }return false;
}

Please Explain If Anyone knows the Right Explanation,After Searching, I couldn't find the Appropriate justification for it.

CodePudding user response：

Your first code says

• If there are at least two unequal characters, it is not a palindrome
• Otherwise, it is a palindrome (i.e. when all the characters "match")

Your second code says

• If there are at least two equal characters, it is a palindrome
• Otherwise, it is not a palindrome (i.e. when none of the characters "match")

If you can't see the difference between them, I suggest you think about it until you do; negation is much trickier than people think.

Another way to see the problem is to notice that in order to be sure that something is a palindrome, you must look at all the characters, so you can't return true before you have done that.
On the other hand, in order to be sure that something isn't a palindrome, you only need to find a single mismatch, so you can return false as soon as you do.

CodePudding user response：

your second code returns true even if only two characters match, like asda will return true but it's not a palindrome neither is bssw but it returns true. You need to compare every characters, if you want to do in the second way then:

bool is_palindrome(string s,int start,int end){
    while(start<=end){
        if(s[start  ]==s[end--]){
            continue;
        }
        else { 
            return false;
       }
    }
    return true;
}