Calculate Money in Leetcode Bank - Practice Coding Problems

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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Understanding the Visualization

Input

Number of days n

Pattern

Weekly deposits: Mon+1, Tue+1, ..., Sun+1, then next Monday+1

Output

Total money saved after n days

Key Takeaway

🎯 Key Insight: Each week follows the same pattern but with Monday amount incrementing by $1

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

Approach	Time	Space	Notes
✓ Mathematical Formula	O(1)	O(1)	Calculate complete weeks with formula, then add remaining days
Day by Day Simulation	O(n)	O(1)	Simulate each day from 1 to n, tracking current amount and day of week

Mathematical Formula — Algorithm Steps

Calculate number of complete weeks
Use formula for sum of complete weeks
Add contribution from remaining days in partial week

Visualization

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Step-by-Step Walkthrough

Complete Weeks

Calculate sum using arithmetic series formula

Remaining Days

Add partial week contribution

Final Sum

Combine both parts for total

Code -

solution.c — C

#include <stdio.h>

int solution(int n) {
    int completeWeeks = n / 7;
    int remainingDays = n % 7;
    
    int total = 0;
    
    // Sum for complete weeks
    if (completeWeeks > 0) {
        total += completeWeeks * 28 + 7 * completeWeeks * (completeWeeks - 1) / 2;
    }
    
    // Add remaining days
    if (remainingDays > 0) {
        int mondayAmount = completeWeeks + 1;
        total += remainingDays * mondayAmount + remainingDays * (remainingDays - 1) / 2;
    }
    
    return total;
}

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

Time & Space Complexity

Time Complexity

⏱️

O(1)

Uses mathematical formulas with constant time operations

✓ Linear Growth

Space Complexity

O(1)

Only uses a few variables for calculations

✓ Linear Space

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Smart Actions

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Mathematical Formula — Algorithm Steps

Visualization

Code -

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