New Problem Solution -"Calculate Money in Leetcode Bank"

Author: haoel

README.md

@@ -61,6 +61,7 @@ LeetCode
 |1733|[Minimum Number of People to Teach](https://leetcode.com/problems/minimum-number-of-people-to-teach/) | [C++](./algorithms/cpp/minimumNumberOfPeopleToTeach/MinimumNumberOfPeopleToTeach.cpp)|Medium|
 |1732|[Find the Highest Altitude](https://leetcode.com/problems/find-the-highest-altitude/) | [C++](./algorithms/cpp/findTheHighestAltitude/FindTheHighestAltitude.cpp)|Easy|
 |1725|[Number Of Rectangles That Can Form The Largest Square](https://leetcode.com/problems/number-of-rectangles-that-can-form-the-largest-square/) | [C++](./algorithms/cpp/numberOfRectanglesThatCanFormTheLargestSquare/NumberOfRectanglesThatCanFormTheLargestSquare.cpp)|Easy|
+|1716|[Calculate Money in Leetcode Bank](https://leetcode.com/problems/calculate-money-in-leetcode-bank/) | [C++](./algorithms/cpp/calculateMoneyInLeetcodeBank/CalculateMoneyInLeetcodeBank.cpp)|Easy|
 |1625|[Lexicographically Smallest String After Applying Operations](https://leetcode.com/problems/lexicographically-smallest-string-after-applying-operations/) | [C++](./algorithms/cpp/lexicographicallySmallestStringAfterApplyingOperations/LexicographicallySmallestStringAfterApplyingOperations.cpp)|Medium|
 |1624|[Largest Substring Between Two Equal Characters](https://leetcode.com/problems/largest-substring-between-two-equal-characters/) | [C++](./algorithms/cpp/largestSubstringBetweenTwoEqualCharacters/LargestSubstringBetweenTwoEqualCharacters.cpp)|Easy|
 |1605|[Find Valid Matrix Given Row and Column Sums](https://leetcode.com/problems/find-valid-matrix-given-row-and-column-sums/) | [C++](./algorithms/cpp/FindValidMatrixGivenRowAndColumnSums/FindValidMatrixGivenRowAndColumnSums.cpp)|Medium|

algorithms/cpp/calculateMoneyInLeetcodeBank/CalculateMoneyInLeetcodeBank.cpp

@@ -0,0 +1,67 @@
+// Source : https://leetcode.com/problems/calculate-money-in-leetcode-bank/
+// Author : Hao Chen
+// Date   : 2021-03-28
+
+/***************************************************************************************************** 
+ *
+ * Hercy wants to save money for his first car. He puts money in the Leetcode bank every day.
+ * 
+ * He starts by putting in $1 on Monday, the first day. Every day from Tuesday to Sunday, he will put 
+ * in $1 more than the day before. On every subsequent Monday, he will put in $1 more than the 
+ * previous Monday. 
+ * 
+ * Given n, return the total amount of money he will have in the Leetcode bank at the end of the n^th 
+ * day.
+ * 
+ * Example 1:
+ * 
+ * Input: n = 4
+ * Output: 10
+ * Explanation: After the 4^th day, the total is 1 + 2 + 3 + 4 = 10.
+ * 
+ * Example 2:
+ * 
+ * Input: n = 10
+ * Output: 37
+ * Explanation: After the 10^th day, the total is (1 + 2 + 3 + 4 + 5 + 6 + 7) + (2 + 3 + 4) = 37. 
+ * Notice that on the 2^nd Monday, Hercy only puts in $2.
+ * 
+ * Example 3:
+ * 
+ * Input: n = 20
+ * Output: 96
+ * Explanation: After the 20^th day, the total is (1 + 2 + 3 + 4 + 5 + 6 + 7) + (2 + 3 + 4 + 5 + 6 + 7 
+ * + 8) + (3 + 4 + 5 + 6 + 7 + 8) = 96.
+ * 
+ * Constraints:
+ * 
+ * 	1 <= n <= 1000
+ ******************************************************************************************************/
+
+class Solution {
+public:
+    int totalMoney(int n) {
+        int weeks = n / 7;
+        int days = n % 7;
+        
+        int m = 1 + 2 + 3 + 4 + 5 + 6 + 7;
+        // we know
+        //     week1 = 0*7 + m
+        //     week2 = 1*7 + m 
+        //     week3 = 2*7 + m 
+        //     weekn = (n-1)*7 + m
+        // So, 
+        //     week1 + week2 + week3 + ....+ weekn 
+        //   = n*m + 7*(0+1+2+..n-1)
+        //   = n*m + 7*(n-1)*n/2
+        
+        int money = weeks*m + 7 * (weeks-1) * weeks / 2;
+        
+        // for the rest days
+        // every day needs to add  previous `weeks * 1`, it is  days* weeks
+        // then add from 1 to days
+        money += (days*weeks + days*(days+1)/2);
+        
+        return money;
+    }
+};