fix find_the_difference (#30)

Author: 0xmountaintop

README.md

@@ -66,7 +66,7 @@ cargo test
 0318| [Maximum Product of Word Lengths](https://leetcode.com/problems/maximum-product-of-word-lengths/) | [C++](https://github.com/kamyu104/LeetCode-Solutions/blob/master/C++/maximum-product-of-word-lengths.cpp) [Python](https://github.com/kamyu104/LeetCode-Solutions/blob/master/Python/maximum-product-of-word-lengths.py) <br>------</br> [Rust](./kamyu104/src/maximum_product_of_word_lengths.rs) | _O(n)_ ~ _O(n^2)_ |  _O(n)_ | Medium         || Bit Manipulation, Counting Sort, Pruning|
 0342 | [Power of Four](https://leetcode.com/problems/power-of-four/) | [C++](https://github.com/kamyu104/LeetCode-Solutions/blob/master/C++/power-of-four.cpp) [Python](https://github.com/kamyu104/LeetCode-Solutions/blob/master/Python/power-of-four.py) <br>------</br> [Rust](./kamyu104/src/power_of_four.rs) | _O(1)_ | _O(1)_ | Easy | |
 0371 | [Sum of Two Integers](https://leetcode.com/problems/sum-of-two-integers/) | [C++](https://github.com/kamyu104/LeetCode-Solutions/blob/master/C++/sum-of-two-integers.cpp) [Python](https://github.com/kamyu104/LeetCode-Solutions/blob/master/Python/sum-of-two-integers.py) <br>------</br> [Rust](./kamyu104/src/sum_of_two_integers.rs) | _O(1)_ | _O(1)_ | Easy | LintCode |
-0389 | [~~Find the Difference~~](https://leetcode.com/problems/find-the-difference/) | [C++](https://github.com/kamyu104/LeetCode-Solutions/blob/master/C++/find-the-difference.cpp) [Python](https://github.com/kamyu104/LeetCode-Solutions/blob/master/Python/find-the-difference.py) <br>------</br> [Rust](./kamyu104/src/find_the_difference.rs) | _O(n)_ | _O(1)_ | Easy | |
+0389 | [Find the Difference](https://leetcode.com/problems/find-the-difference/) | [C++](https://github.com/kamyu104/LeetCode-Solutions/blob/master/C++/find-the-difference.cpp) [Python](https://github.com/kamyu104/LeetCode-Solutions/blob/master/Python/find-the-difference.py) <br>------</br> [Rust](./kamyu104/src/find_the_difference.rs) | _O(n)_ | _O(1)_ | Easy | |
 0393 | [UTF-8 Validation](https://leetcode.com/problems/utf-8-validation/) | [C++](https://github.com/kamyu104/LeetCode-Solutions/blob/master/C++/utf-8-validation.cpp) [Python](https://github.com/kamyu104/LeetCode-Solutions/blob/master/Python/utf-8-validation.py) <br>------</br> [Rust](./kamyu104/src/utf_8_validation.rs) | _O(n)_ | _O(1)_ | Medium | |
 
 <!-- 

kamyu104/src/find_the_difference.rs

@@ -1,17 +1,74 @@
-// pub struct Solution {}
-// impl Solution {
+use std::collections::HashSet;
+
+// Time:  O(n)
+// Space: O(1)
+
+// pub struct Solution1 {}
+// impl Solution1 {
+//     pub fn find_the_difference(s: String, t: String) -> char {
+//         'e'
+//     }
+// }
+
+// pub struct Solution2 {}
+// impl Solution2 {
+//     pub fn find_the_difference(s: String, t: String) -> char {
+//         'e'
+//     }
+// }
+
+// https://github.com/kamyu104/LeetCode-Solutions/blob/master/Python/find-the-difference.py#L18
+pub struct Solution3 {}
+impl Solution3 {
+    pub fn find_the_difference(s: String, t: String) -> char {
+        let s_chars: HashSet<char> = s.chars().collect();
+        let mut t_chars: HashSet<char> = t.chars().collect();
+        s_chars.iter().for_each(|c| {
+            t_chars.remove(c);
+        });
+        t_chars.into_iter().collect::<Vec<char>>()[0]
+    }
+}
+
+// pub struct Solution4 {}
+// impl Solution4 {
+//     pub fn find_the_difference(s: String, t: String) -> char {
+//         'e'
+//     }
+// }
+
+// https://github.com/kamyu104/LeetCode-Solutions/blob/master/Python/find-the-difference.py#L33
+pub struct Solution5 {}
+impl Solution5 {
+    pub fn find_the_difference(s: String, t: String) -> char {
+        let s_chars: HashSet<char> = s.chars().collect();
+        let t_chars: HashSet<char> = t.chars().collect();
+        // or https://doc.rust-lang.org/std/collections/struct.HashSet.html#method.symmetric_difference
+        let diff: HashSet<&char> = t_chars.difference(&s_chars).collect();
+        *(diff.into_iter().collect::<Vec<&char>>()[0])
+    }
+}
+
+// pub struct Solution6 {}
+// impl Solution6 {
 //     pub fn find_the_difference(s: String, t: String) -> char {
 //         'e'
 //     }
 // }
 
 #[cfg(test)]
 mod tests {
-    // Note this useful idiom: importing names from outer (for mod tests) scope.
     use super::*;
 
     #[test]
     fn test_find_the_difference() {
-        // assert_eq!(Solution::find_the_difference("abcd".to_string(), "abcde".to_string()), 'e');
+        assert_eq!(
+            Solution3::find_the_difference("abcd".to_string(), "abcde".to_string()),
+            'e'
+        );
+        assert_eq!(
+            Solution5::find_the_difference("abcd".to_string(), "abcde".to_string()),
+            'e'
+        );
     }
 }