680. Valid Palindrome II

本文介绍了一种算法,通过使用双指针遍历字符串并检查其是否满足回文条件,即使在允许删除一个字符的情况下也能判断字符串是否为有效回文。提供了详细的实现思路及C++代码示例。

当字符串中最多可删除一个元素时,考虑字符串是否仍然满足回文要求:双指针遍历,逐一匹配相应位置的字符;在不匹配的位置,令左指针右移一位或者右指针左移一位(删除一个字符),在分别考虑当下的情况是否满足回文要求。

class Solution {
public:
    bool validPalindrome(string s) {
        int left=0,right=s.size()-1;
        int flag=1;
        while(left<right)
        {
            if(s[left]==s[right])
            {
                left++;
                right--;
            }
            else
            {
                return valid(s,left,right-1)||valid(s,left+1,right);
            }
        }
        return 1;
    }
    bool valid(string s,int begin,int end)
    {
        while(begin<end)
        {
            if(s[begin]==s[end])
            {
                begin++;
                end--;
            }
            else
                return 0;
        }
        return 1;
    }
};

 

### XTUOJ Perfect Palindrome Problem Analysis For the **Perfect Palindrome** problem on the XTUOJ platform, understanding palindromes and string manipulation algorithms plays a crucial role. A palindrome refers to a word, phrase, number, or other sequences of characters which reads the same backward as forward[^1]. The challenge typically involves checking whether a given string meets specific conditions to be considered a perfect palindrome. In many similar problems, preprocessing steps such as converting all letters into lowercase (or uppercase) can simplify subsequent checks by ensuring case insensitivity during comparison operations. Additionally, removing non-alphanumeric characters ensures that only relevant symbols participate in determining if the sequence forms a valid palindrome[^2]. To determine if a string is a perfect palindrome, one approach iterates from both ends towards the center while comparing corresponding elements until reaching the midpoint without encountering mismatches: ```python def is_perfect_palindrome(s): cleaned_string = ''.join(char.lower() for char in s if char.isalnum()) left_index = 0 right_index = len(cleaned_string) - 1 while left_index < right_index: if cleaned_string[left_index] != cleaned_string[right_index]: return False left_index += 1 right_index -= 1 return True ``` This function first creates `cleaned_string`, stripping away any irrelevant characters and normalizing cases. Then through iteration with two pointers moving inward simultaneously (`left_index` starting at position 0 and `right_index` initially set to the last index), comparisons occur between pairs of opposing positions within the processed input string. If every pair matches perfectly throughout this process, then the original string qualifies as a "perfect palindrome".
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