feat: Add solution for happy number problem

Author: pragusga25

easy/happy-number.test.ts

@@ -0,0 +1,33 @@
+// Problem: https://leetcode.com/problems/happy-number/
+// Doc: https://leetcode.com/problems/happy-number/solutions/5471203/determining-happy-numbers-efficiently/
+const isHappy = (n: number): boolean => {
+  // I assume that we cannot use string conversion to make it challenging
+  const visited: Record<number, boolean> = {};
+  while (true) {
+    if (n === 1) return true;
+    if (n in visited) return false;
+    visited[n] = true;
+
+    let sum = 0;
+    let tmp = n;
+    while (tmp > 0) {
+      const remainder = tmp % 10;
+      tmp = Math.floor(tmp / 10);
+      sum += Math.pow(remainder, 2);
+    }
+    n = sum;
+  }
+};
+
+describe('Happy Number', () => {
+  it('should return true if the number is happy', () => {
+    expect(isHappy(19)).toBeTruthy();
+    expect(isHappy(1)).toBeTruthy();
+    expect(isHappy(7)).toBeTruthy();
+  });
+
+  it('should return false if the number is not happy', () => {
+    expect(isHappy(2)).toBeFalsy();
+    expect(isHappy(3)).toBeFalsy();
+  });
+});