Calculate Money in Leetcode Bank

Calculate Money in Leetcode Bank - Problem

Math Easy

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 nth day.

Input & Output

Example 1 — Basic Case

$ Input: n = 4

› Output: 10

💡 Note: Day 1 (Mon): $1, Day 2 (Tue): $2, Day 3 (Wed): $3, Day 4 (Thu): $4. Total: 1+2+3+4 = 10

Example 2 — Complete Week

$ Input: n = 10

› Output: 37

💡 Note: Week 1: $1+$2+$3+$4+$5+$6+$7 = $28. Week 2 (3 days): $2+$3+$4 = $9. Total: 28+9 = 37

Example 3 — Single Day

$ Input: n = 1

› Output: 1

💡 Note: Only Monday of first week: $1

Constraints

1 ≤ n ≤ 1000

Visualization

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Asked in

G Google 15 a Amazon 12

The key insight is recognizing the weekly pattern where each week starts with $1 more than the previous Monday. Best approach uses mathematical formulas to calculate complete weeks and remaining days efficiently. Time: O(1), Space: O(1)

Common Approaches

✓ Day by Day Simulation

⏱️ Time: O(n) Space: O(1)

Track the current day of the week and the amount deposited each day. For each day, add the appropriate amount based on the week number and day within that week.

Mathematical Formula

⏱️ Time: O(1) Space: O(1)

Use arithmetic series formula to calculate sum for complete weeks, then add the remaining partial week. Each complete week contributes 28 + 7*weekNumber dollars.

Day by Day Simulation — Algorithm Steps

Start with $1 on day 1 (Monday)
For each subsequent day, increment amount by $1 within the week
When starting a new week (Monday), add $1 to the Monday amount

Visualization

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Step-by-Step Walkthrough

Week 1

Mon=$1, Tue=$2, Wed=$3, Thu=$4, Fri=$5, Sat=$6, Sun=$7

Week 2

Mon=$2, Tue=$3, Wed=$4, Thu=$5, Fri=$6, Sat=$7, Sun=$8

Sum Total

Add all daily amounts up to day n

Code -

solution.c — C

#include <stdio.h>

int solution(int n) {
    int total = 0;
    int mondayAmount = 1;
    
    for (int day = 1; day <= n; day++) {
        int dayOfWeek = (day - 1) % 7; // 0=Monday, 1=Tuesday, ..., 6=Sunday
        int currentAmount = mondayAmount + dayOfWeek;
        total += currentAmount;
        
        // If this day is Sunday and not the last day, increment Monday amount
        if (dayOfWeek == 6 && day < n) {
            mondayAmount++;
        }
    }
    
    return total;
}

int main() {
    int n;
    scanf("%d", &n);
    printf("%d\n", solution(n));
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Iterates through each day from 1 to n once

✓ Linear Growth

Space Complexity

O(1)

Only uses a few variables to track current state

✓ Linear Space

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Medium Frequency

~15 min Avg. Time

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Smart Actions

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Input & Output

Constraints

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Related Problems

Common Approaches

Day by Day Simulation — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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