Merge pull request #4 from xchux/feature/125-valid-palindrome

✨ 125. Valid Palindrome

Author: xchux

solutions/125.Valid Palindrome/README.md

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+---
+comments: true
+difficulty: easy
+# Follow `Topics` tags
+tags:
+    - String
+    - Two Pointers
+---
+
+# [125. Valid Palindrome](https://leetcode.com/problems/valid-palindrome/description/)
+
+## Description
+
+A palindrome is a sequence that reads the same forward and backward.
+
+For this problem, a string is considered a palindrome if:
+
+1. All uppercase letters are converted to lowercase.
+2. All non-alphanumeric characters (such as spaces, punctuation, and symbols) are removed.
+3. The remaining characters form a sequence that reads identically from both directions.
+
+Your task is to check whether a given string meets these conditions and return true if it does, otherwise return false.
+
+**Example 1:**
+```
+Input: Was it a car or a cat I saw?
+Output: true
+Explanation: "wasitacaroracatisaw" is a palindrome.
+```
+
+**Example 2:**
+```
+Input: How are you?
+Output: false
+Explanation: "howareyou" is not a palindrome.
+```
+
+**Constraints:**
+
+`1 <= s.length <= 2 * 10^5`
+
+`s consists only of printable ASCII characters.`
+
+## Solution
+
+Intuitively remove non-alphanumeric and change others to lowercase.
+
+```java
+class Solution {
+    public boolean isPalindrome(String s) {
+        s = s.toLowerCase().replaceAll("[^a-z0-9]", "");
+        int i = 0, j = s.length() - 1;
+        while (i < j) {
+            if (s.charAt(i++) != s.charAt(j--)) return false;
+        }
+        return true;
+    }
+}
+```
+
+## Complexity
+
+- Time complexity: $$O(n)$$
+<!-- Add time complexity here, e.g. $$O(n)$$ -->
+
+- Space complexity: $$O(n)$$
+<!-- Add space complexity here, e.g. $$O(n)$$ -->

solutions/125.Valid Palindrome/Solution.java

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+class Solution {
+    public boolean isPalindrome(String s) {
+        s = s.toLowerCase().replaceAll("[^a-z0-9]", "");
+        int i = 0, j = s.length() - 1;
+        while (i < j) {
+            if (s.charAt(i++) != s.charAt(j--)) return false;
+        }
+        return true;
+    }
+}