[Hacker Rank] Interview Preparation Kit: Hash Tables: Ice Cream Parlor. Solved ✅.

Author: Gonzalo Diaz

docs/hackerrank/interview_preparation_kit/search/ctci-ice-cream-parlor-solution-notes.md

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+# [Hash Tables: Ice Cream Parlor](https://www.hackerrank.com/challenges/ctci-ice-cream-parlor)
+
+- Difficulty:  `#medium`
+- Category: `#ProblemSolvingIntermediate`
+
+## Failed tries
+
+### Brute force
+
+First attempt: Brute force. Complexity O(n^2)
+Fails to pass additional test cases due to excessive time.
+
+### Binary search
+
+This attempt use the editorial suggestion: do a binary search.
+
+For each flavor of ice cream on the cost list, the searched one
+is "trimmed" in a search from half (then half of half and so on) of the list.
+Complexity O(log n)
+
+For some reason that I have not reviewed in detail,
+it works with the test cases in the examples,
+but fails in the result with all the hidden test cases.
+
+The preliminary guess is that the "edges" may be poorly thought out,
+and it may also not properly account for cases where there may be
+"repeated" values in the search.
+
+## Working solution
+
+This solution has a complexity of O(N).
+It is based on a very simple idea.
+
+A dictionary (key-value object) is created.
+
+Then for each item (current ice cream):
+
+- The difference between the budget and the current item is calculated.
+    It tells us the value of the element to search for.
+
+- We look for the element if it is in the "cache" (dictionary).
+    We look for the value of the current element among the cache keys.
+
+- If the element is NOT found in the dictionary,
+    then we store the currently searched element in the cache,
+    using the element's value as the cache key and the position
+    (what we care about returning) as the cache value.
+
+If the element is found, then we return the key of the current element
+and the key of the element found in the cache.
+
+This mechanism also returns the smallest element that matches,
+in the case of repeated values (because the search is from start to finish,
+without reordering). In the case of passing by repeated values,
+the current element to be stored in the cache,
+steps on the previous position value with the new one,
+but since it is not the searched value, it is irrelevant.
+
+- Source: [Hackerrank Hash Tables - Ice Cream Parlor Python solution](https://medium.com/@xww0701/hackerrank-hash-tables-ice-cream-parlor-python-solution-fac434523ec7)

docs/hackerrank/interview_preparation_kit/search/ctci-ice-cream-parlor.md

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+# [Hash Tables: Ice Cream Parlor](https://www.hackerrank.com/challenges/ctci-ice-cream-parlor)
+
+- Difficulty:  `#medium`
+- Category: `#ProblemSolvingIntermediate`
+
+Each time Sunny and Johnny take a trip to the Ice Cream Parlor,
+they pool their money to buy ice cream.
+On any given day, the parlor offers a line of flavors.
+Each flavor has a cost associated with it.
+
+Given the value of `money` and the `cost` of each flavor for `t`
+trips to the Ice Cream Parlor, help Sunny and Johnny choose two
+distinct flavors such that they spend their entire pool of money during each visit.
+ID numbers are the 1-based index number associated with a `cost`.
+For each trip to the parlor, print the ID numbers for the two
+types of ice cream that Sunny and Johnny purchase as
+two space-separated integers on a new line. You must print the smaller
+ID first and the larger ID second.
+
+## Example
+
+`cost = [2, 1, 3, 5, 6]`
+
+`money = 5`
+
+They would purchase flavor ID's `1` and `3` for a cost of `2 + 3 = 5`.
+Use 1-based indexing for your response.
+
+**Note**:
+
+- Two ice creams having unique IDs  and  may have the same cost
+(i.e., $ cost[i] \equiv cost[j] $).
+
+- There will always be a unique solution.
+
+## Function Description
+
+Complete the function whatFlavors in the editor below.
+
+whatFlavors has the following parameter(s):
+
+- `int cost[n]`: the prices for each flavor
+- `int money`: the amount of money they have to spend
+
+## Prints
+
+- `int int`: the indices of the two flavors they will purchase as
+    two space-separated integers on a line
+
+## Input Format
+
+The first line contains an integer, `t`, the number of trips to the ice cream parlor.
+
+Each of the next  sets of  lines is as follows:
+
+- The first line contains `money`.
+- The second line contains an integer, `n`, the size of the array `cost`.
+- The third line contains `n` space-separated integers denoting the `cost[i]`.
+
+## Constraints
+
+- $ 1 \leq t \leq 50$
+- $ 2 \leq money \leq 10^9 $
+- $ 2 \leq n \leq 5 * 10^4 $
+- $ 1 \leq cost[i] \leq 10^9 $
+
+## Sample Input
+
+```text
+STDIN       Function
+-----       --------
+2           t = 2
+4           money = 4
+5           cost[] size n = 5
+1 4 5 3 2   cost = [1, 4, 5, 3, 2]
+4           money = 4
+4           cost[] size n = 4
+2 2 4 3     cost = [2, 2, 4, 3]
+```
+
+## Sample Output
+
+```text
+1 4
+1 2
+```
+
+## Explanation
+
+Sunny and Johnny make the following two trips to the parlor:
+
+1. The first time, they pool together $ money = 4 $ dollars.
+    There are five flavors available that day and
+    flavors `1` and `4` have a total cost of `1 + 3 = 4`.
+2. The second time, they pool together `money = 4` dollars.
+    There are four flavors available that day and
+    flavors `1` and `2` have a total cost of `2 + 2 = 4`.

src/hackerrank/interview_preparation_kit/search/ctci_ice_cream_parlor.border_testcases.json

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+[
+  {
+    "title": "Sample Test Case 0",
+    "tests": [
+      {
+        "costs": [1, 4, 5, 3, 2],
+        "money": 90,
+        "expected": []
+      }
+    ]
+  }
+]

src/hackerrank/interview_preparation_kit/search/ctci_ice_cream_parlor.test.ts

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+import { describe, expect, it } from '@jest/globals';
+import { logger as console } from '../../../logger';
+
+import { whatFlavors, whatFlavorsCompute } from './ctci_ice_cream_parlor';
+import TEST_CASES from './ctci_ice_cream_parlor.testcases.json';
+import TEST_CASES_BORDER_CASES from './ctci_ice_cream_parlor.border_testcases.json';
+
+describe('ctci_ice_cream_parlor', () => {
+  it('whatFlavorsCompute test cases', () => {
+    expect.assertions(10);
+
+    TEST_CASES.forEach((testSet) => {
+      testSet?.tests.forEach((test) => {
+        const answer = whatFlavorsCompute(test.costs, test.money);
+
+        console.debug(
+          `whatFlavorsCompute(${test.costs}, ${test.money}) solution found: ${answer}`
+        );
+
+        expect(answer).toStrictEqual(test.expected);
+        expect(whatFlavors(test.costs, test.money)).toBeUndefined();
+      });
+    });
+  });
+
+  it('whatFlavors border test cases', () => {
+    expect.assertions(2);
+
+    TEST_CASES_BORDER_CASES.forEach((testSet) => {
+      testSet?.tests.forEach((test) => {
+        expect(whatFlavorsCompute(test.costs, test.money)).toStrictEqual(
+          test.expected
+        );
+        expect(whatFlavors(test.costs, test.money)).toBeUndefined();
+      });
+    });
+  });
+
+  it('whatFlavors test caller function', () => {
+    expect.assertions(1);
+
+    const cost: number[] = [];
+    const money: number = 100;
+
+    expect(whatFlavors(cost, money)).toBeUndefined();
+  });
+});

src/hackerrank/interview_preparation_kit/search/ctci_ice_cream_parlor.testcases.json

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+[
+  {
+    "title": "Sample Test Case 0",
+    "tests": [
+      {
+        "costs": [1, 4, 5, 3, 2],
+        "money": 4,
+        "expected": [1, 4]
+      },
+      {
+        "costs": [2, 2, 4, 3],
+        "money": 4,
+        "expected": [1, 2]
+      }
+    ]
+  },
+  {
+    "title": "Sample Test Case 1",
+    "tests": [
+      {
+        "costs": [1, 2, 3, 5, 6],
+        "money": 5,
+        "expected": [2, 3]
+      }
+    ]
+  },
+  {
+    "title": "Sample Test Case 2",
+    "tests": [
+      {
+        "costs": [4, 3, 2, 5, 7],
+        "money": 8,
+        "expected": [2, 4]
+      },
+      {
+        "costs": [7, 2, 5, 4, 11],
+        "money": 12,
+        "expected": [1, 3]
+      }
+    ]
+  }
+]

src/hackerrank/interview_preparation_kit/search/ctci_ice_cream_parlor.ts

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+/**
+ * @link Problem definition [[docs/hackerrank/interview_preparation_kit/search/ctci-ice-cream-parlor.md]]
+ */
+
+export function whatFlavorsCompute(
+  cost: number[],
+  money: number
+): number[] | null {
+  const cache: Record<number, number> = {};
+
+  for (const [key, price] of Object.entries(cost)) {
+    const i = parseInt(key);
+    const diff = money - price;
+    const cacheKeys = new Set(Object.keys(cache));
+
+    if (cacheKeys.has(diff.toString())) {
+      return [i + 1, cache[diff] + 1].sort((a, b) => a - b);
+    }
+
+    cache[price] = i;
+  }
+
+  return [];
+}
+
+export function whatFlavors(cost: number[], money: number): void {
+  console.log(whatFlavorsCompute(cost, money)?.join(' '));
+}
+
+export default { whatFlavorsCompute, whatFlavors };