Added happy number solution.

README.md

@@ -171,6 +171,7 @@ programming challenges completed in other languages.
 - [Ransom Note](https://leetcode.com/problems/ransom-note)
 - [Word Pattern](https://leetcode.com/problems/word-pattern)
 - [Valid Anagram](https://leetcode.com/problems/valid-anagram)
+- [Happy Number](https://leetcode.com/problems/happy-number)
 
 #### Medium
 

src/leetcode/happy_number.rs

@@ -0,0 +1,45 @@
+use std::collections::HashSet;
+
+fn get_digit_square_sum(mut n: u64) -> u64 {
+    let mut sum = 0;
+    while n > 0 {
+        sum += (n % 10).pow(2);
+        n /= 10;
+    }
+    sum
+}
+
+pub fn is_happy(n: i32) -> bool {
+    let mut seen = HashSet::new();
+    let mut curr = get_digit_square_sum(n as u64);
+    while seen.insert(curr) {
+        if curr == 1 {
+            return true;
+        } 
+        curr = get_digit_square_sum(curr);
+    }
+    false
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn identifies_happy_numbers() {
+        assert!(is_happy(19))
+    }
+
+    #[test]
+    fn identifies_unhappy_numbers() {
+        assert!(!is_happy(2))
+    }
+
+    #[test]
+    fn calculates_sum_square_of_digits() {
+        assert_eq!(82, get_digit_square_sum(19));
+        assert_eq!(68, get_digit_square_sum(82));
+        assert_eq!(100, get_digit_square_sum(68));
+        assert_eq!(1, get_digit_square_sum(100))
+    }
+}

src/leetcode/mod.rs

@@ -47,6 +47,7 @@ mod grid_illumination;
 mod group_anagrams;
 mod guess_number;
 mod hamming_distance;
+mod happy_number;
 mod hour_glass_sum;
 mod intersection_two;
 mod invert_tree;