Description (leetcode)

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

submission

// the total money saved each week is an
// arithmetic sequence, the money saved each
// day within a week is also an arithmetic
// sequence, hence we have the following
// formula.
impl Solution {
    pub fn total_money(n: i32) -> i32 {
        let (w, r) = (n / 7, n % 7);
        w * 21 + w * (w + 1) * 7 / 2 + // total weeks
            w * r + r * (r + 1) / 2 // rest days
    }
}