add 125

Author: andavid

README.md

@@ -35,6 +35,8 @@
 | 7    | [Reverse Integer][007]                      |
 | 387  | [First Unique Character in a String][387]   |
 | 242  | [Valid Anagram][242]                        |
+| 125  | [Valid Palindrome][125]                     |
+
 
 **其他**
 
@@ -64,3 +66,4 @@
 [007]: https://github.com/andavid/leetcode-java/blob/master/note/007/README.md
 [387]: https://github.com/andavid/leetcode-java/blob/master/note/387/README.md
 [242]: https://github.com/andavid/leetcode-java/blob/master/note/242/README.md
+[125]: https://github.com/andavid/leetcode-java/blob/master/note/125/README.md

note/125/README.md

@@ -0,0 +1,69 @@
+# [Valid Palindrome][title]
+
+## Description
+
+Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
+
+For example,
+"A man, a plan, a canal: Panama" is a palindrome.
+"race a car" is not a palindrome.
+
+**Note:**
+Have you consider that the string might be empty? This is a good question to ask during an interview.
+
+For the purpose of this problem, we define empty string as valid palindrome.
+
+## 思路
+
+判断字符串是否是回文字符串，忽略除了数字和大小写字母之外的字符。
+使用头和尾两个指针往中间扫描，遇到非数字和字母的字符跳过，比较两个字符是否相等（忽略大小写）。
+
+## [完整代码][src]
+
+```java
+class Solution {
+  public boolean isPalindrome(String s) {
+    if (s == null || s.length() <= 1) {
+      return true;
+    }
+
+    int i = 0;
+    int j = s.length() - 1;
+
+    while (i < j) {
+      char lc = s.charAt(i);
+      char rc = s.charAt(j);
+
+      if (!isAlphaNumeric(lc)) {
+        i++;
+        continue;
+      }
+
+      if (!isAlphaNumeric(rc)) {
+        j--;
+        continue;
+      }
+
+      if (toLowerCase(lc) != toLowerCase(rc)) {
+        return false;
+      }
+
+      i++;
+      j--;
+    }
+
+    return true;
+  }
+
+  public boolean isAlphaNumeric(char c) {
+    return (c >= 48 && c <= 57) || (c >= 65 && c <= 90) || (c >= 97 && c <= 122);
+  }
+
+  public int toLowerCase(char c) {
+    return (c >= 97 && c <= 122) ? c - 32 : c;
+  }
+}
+```
+
+[title]: https://leetcode.com/problems/valid-palindrome
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_125/Solution.java

src/com/andavid/leetcode/_125/Solution.java

@@ -0,0 +1,48 @@
+class Solution {
+  public boolean isPalindrome(String s) {
+    if (s == null || s.length() <= 1) {
+      return true;
+    }
+
+    int i = 0;
+    int j = s.length() - 1;
+
+    while (i < j) {
+      char lc = s.charAt(i);
+      char rc = s.charAt(j);
+
+      if (!isAlphaNumeric(lc)) {
+        i++;
+        continue;
+      }
+
+      if (!isAlphaNumeric(rc)) {
+        j--;
+        continue;
+      }
+
+      if (toLowerCase(lc) != toLowerCase(rc)) {
+        return false;
+      }
+
+      i++;
+      j--;
+    }
+
+    return true;
+  }
+
+  public boolean isAlphaNumeric(char c) {
+    return (c >= 48 && c <= 57) || (c >= 65 && c <= 90) || (c >= 97 && c <= 122);
+  }
+
+  public int toLowerCase(char c) {
+    return (c >= 97 && c <= 122) ? c - 32 : c;
+  }
+
+  public static void main(String[] args) {
+    Solution solution = new Solution();
+    System.out.println(solution.isPalindrome("A man, a plan, a canal: Panama"));
+    System.out.println(solution.isPalindrome("0P"));
+  }
+}