add 242

Author: andavid

README.md

@@ -34,7 +34,7 @@
 | 344  | [Reverse String][344]                       |
 | 7    | [Reverse Integer][007]                      |
 | 387  | [First Unique Character in a String][387]   |
-
+| 242  | [Valid Anagram][242]                        |
 
 **其他**
 
@@ -63,3 +63,4 @@
 [344]: https://github.com/andavid/leetcode-java/blob/master/note/344/README.md
 [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

note/242/README.md

@@ -0,0 +1,70 @@
+# [Valid Anagram][title]
+
+## Description
+
+Given two strings s and t, write a function to determine if t is an anagram of s.
+
+For example,
+s = "anagram", t = "nagaram", return true.
+s = "rat", t = "car", return false.
+
+**Note:**
+You may assume the string contains only lowercase alphabets.
+
+**Follow up:**
+What if the inputs contain unicode characters? How would you adapt your solution to such case?
+
+## 思路一
+题意是判断两个字符串是否是同字母异序词，即由相同字母变换顺序后组成的新词。
+首先判断长度是否一致，如果不一致的话肯定不是。
+然后将字符串转成字符数组，对字符数组进行排序，比较两个排序后的字符数组是否完全一致。
+
+## [完整代码][src]
+
+```java
+class Solution {
+  public boolean isAnagram(String s, String t) {
+    if (s == null || t == null || s.length() != t.length()) {
+      return false;
+    }
+    char[] chs = s.toCharArray();
+    char[] cht = t.toCharArray();
+    Arrays.sort(chs);
+    Arrays.sort(cht);
+    return (new String(chs)).equals(new String(cht));
+  }
+}
+```
+
+## 思路二
+由于题目中字符串只包含字母，因此可以使用一个数组记录26个字母出现的次数，遍历第一个字符串时次数加1，遍历第二个字符串时出现次数减1，一旦发现某个字符出现次数小于等于0时，说明该字符要么是从未出现过的字符，要么是出现次数超过第一个字符串，肯定不是同字母异序词。
+
+## [完整代码][src]
+
+```java
+class Solution {
+  public boolean isAnagram(String s, String t) {
+    if (s == null || t == null || s.length() != t.length()) {
+      return false;
+    }
+
+    int[] count = new int[26];
+
+    for (char ch : s.toCharArray()) {
+      count[ch - 'a']++;
+    }
+
+    for (char ch : t.toCharArray()) {
+      if (count[ch - 'a'] <= 0) {
+        return false;
+      }
+      count[ch - 'a']--;
+    }
+
+    return true;
+  }
+}
+```
+
+[title]: https://leetcode.com/problems/valid-anagram
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_242/Solution.java

src/com/andavid/leetcode/_242/Solution.java

@@ -0,0 +1,37 @@
+import java.util.*;
+
+class Solution {
+  public boolean isAnagram1(String s, String t) {
+    if (s == null || t == null || s.length() != t.length()) {
+      return false;
+    }
+    char[] chs = s.toCharArray();
+    char[] cht = t.toCharArray();
+    Arrays.sort(chs);
+    Arrays.sort(cht);
+    return (new String(chs)).equals(new String(cht));
+  }
+
+  public boolean isAnagram(String s, String t) {
+    if (s == null || t == null || s.length() != t.length()) {
+      return false;
+    }
+    int[] count = new int[26];
+    for (char ch : s.toCharArray()) {
+      count[ch - 'a']++;
+    }
+    for (char ch : t.toCharArray()) {
+      if (count[ch - 'a'] <= 0) {
+        return false;
+      }
+      count[ch - 'a']--;
+    }
+    return true;
+  }
+
+  public static void main(String[] args) {
+    Solution solution = new Solution();
+    System.out.println(solution.isAnagram("anagram", "nagaram"));
+    System.out.println(solution.isAnagram("rat", "cat"));
+  }
+}