LeetCode Entry

1716. Calculate Money in Leetcode Bank

25.10.2025 easy 2025 kotlin rust

Add increasing sum of money, drop weekly gain on mondays

1716. Calculate Money in Leetcode Bank easy blog post substack youtube

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Problem TLDR

Add increasing sum of money, drop weekly gain on mondays #easy

Intuition

Simulate.

    // 0 1 2 3  4  5  6
    // 7 8 9 10 11 12 13
    // 14
    // 1 2 3 4 5 6 7
    // 2 3 4 5 6 7 8   or prev + 7
    // 3 4 5 6 7 8 9   or prev + 7
    // 4 5 6 7 8 9 10  or prev + 7

The O(1) solutino: count weeks and remainder of days. Each week contributes as a sum of base7. Contribution of each bases is w(w-1)/2. Another week contribution is weekly gains, 7*(7+1)/2. They are just w*gains

Approach

• draw the numbers to better understand the laws

Complexity

• Time complexity: \(O(n)\) or O(1)

• Space complexity: \(O(1)\)

Code

// 8ms
    fun totalMoney(n: Int) = (0..<n).sumOf { it % 7 + it / 7 + 1 }

// 0ms
    pub fn total_money(n: i32) -> i32 {
        let w = n/7; let d = n-7*w;
        let fullweeks = 28*w + 7*w*(w-1)/2;
        let taildays = d*(d+1)/2 + w*d;
        fullweeks + taildays
    }