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 nth day.

Example 1:

Input: n = 4
Output: 10
Explanation: After the 4th day, the total is 1 + 2 + 3 + 4 = 10.

Example 2:

Input: n = 10
Output: 37
Explanation: After the 10th day, the total is (1 + 2 + 3 + 4 + 5 + 6 + 7) + (2 + 3 + 4) = 37. Notice that on the 2nd Monday, Hercy only puts in $2.

Example 3:

Input: n = 20
Output: 96
Explanation: After the 20th 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

Solution 1: Simulation

Increase the amount by 1 everyday, the decrease 6 after every sunday.

Time complexity: O(n)
Space complexity: O(1)

// Author: huahua
class Solution {
public:
  int totalMoney(int n) {
    int total = 0;
    for (int d = 0, m = 1; d < n; ++d) {
      total += m++;
      if (d % 7 == 6) m -= 6;
    }
    return total;
  }
};

Could also be solved using Math in O(1)

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