CSES Solutions – Minimizing Coins

Consider a money system consisting of N coins. Each coin has a positive integer value. Your task is to produce a sum of money X using the available coins in such a way that the number of coins is minimal.

Examples:

Input: N = 3, X = 11, coins[] = {1, 5, 7}
Output: 3
Explanation: We need minimum 3 coins to make the sum 11, 5 + 5 + 1 = 11.

Input: N = 3, X = 6, coins[] = {1, 2, 3}
Output: 2
Explanation: We need 2 coins to make sum = 6, 3 + 3 = 6.

Approach: To solve the problem, follow the below idea:

The problem can be solved using Dynamic Programming. We can maintain a dp[] array, such that dp[i] stores the minimum number of coins needed to make sum = i. We can iterate i from 1 to X, and find the minimum number of coins to make sum = i. For any sum i, we assume that the last coin used was the jth coin where j will range from 0 to N – 1. For jth coin, the value will be coins[j], so the minimum number of coins to make sum as i if the last coin used was the jth coin = dp[i – coins[j]] + 1. The formula for the ith sum will be: dp[i] = min(dp[i], dp[i – coins[j]] + 1), for all j from 0 to N – 1

Also, the above formula will be true only when coins[j] >= i and it is possible to make sum = i – coins[j].

Step-by-step algorithm:

• Maintain a dp[] array, such that dp[i] stores the minimum number of coins to make sum = i.
• Initialize dp[0] = 0 as 0 coins are required to make sum = 0.
• Iterate i from 1 to X and calculate minimum number of coins to make sum = i.
• Iterate j over all possible coins assuming the jth coin to be last coin which was selected to make sum = i and update dp[i] with dp[i – coins[j]] + 1 if dp[i – coins[j]] + 1 < dp[i].
• After all the iterations, return the final answer as dp[X].

Below is the implementation of the algorithm:

C++

#include <bits/stdc++.h>
#define ll long long int
#define INF 1000000000
using namespace std;

// Function to find the minimum number of coins to make 
// sum X
ll solve(ll N, ll X, vector<ll>& coins)
{
    // dp[] array such that dp[i] stores the minimum number
    // of coins to make sum = i
    vector<ll> dp(X + 1, INF);
    dp[0] = 0;

    // Iterate over all possible sums from 1 to X
    for (int i = 1; i <= X; i++) {
        // Iterate over all coins
        for (int j = 0; j < N; j++) {
            if (coins[j] > i || dp[i - coins[j]] == INF)
                continue;
            dp[i] = min(dp[i], dp[i - coins[j]] + 1);
        }
    }
    // If it is possible to make sum = X, return dp[X]
    if (dp[X] != INF)
        return dp[X];

    // If it is impossible to make sum = X, return -1
    return -1;
}

int main()
{
    // Sample Input
    ll N = 3, X = 6;
    vector<ll> coins = { 1, 2, 3 };
    
    cout << solve(N, X, coins) << "\n";
}

Java

import java.util.*;

public class MinCoinsToMakeSum {
    static final int INF = 1000000000;

    // Function to find the minimum number of coins to make sum X
    static long solve(int N, int X, List<Integer> coins) {
        // dp[] array such that dp[i] stores the minimum number
        // of coins to make sum = i
        long[] dp = new long[X + 1];
        Arrays.fill(dp, INF);
        dp[0] = 0;

        // Iterate over all possible sums from 1 to X
        for (int i = 1; i <= X; i++) {
            // Iterate over all coins
            for (int j = 0; j < N; j++) {
                if (coins.get(j) > i || dp[i - coins.get(j)] == INF) {
                    continue;
                }
                dp[i] = Math.min(dp[i], dp[i - coins.get(j)] + 1);
            }
        }

        // If it is possible to make sum = X, return dp[X]
        if (dp[X] != INF) {
            return dp[X];
        }

        // If it is impossible to make sum = X, return -1
        return -1;
    }

    public static void main(String[] args) {
        // Sample Input
        int N = 3, X = 6;
        List<Integer> coins = Arrays.asList(1, 2, 3);

        System.out.println(solve(N, X, coins));
    }
}

// This code is contributed by shivamgupta0987654321

Python

# Function to find the minimum number of coins to make 
# sum X
def solve(N, X, coins):
    # dp[] array such that dp[i] stores the minimum number
    # of coins to make sum = i
    dp = [float('inf')] * (X + 1)
    dp[0] = 0

    # Iterate over all possible sums from 1 to X
    for i in range(1, X + 1):
        # Iterate over all coins
        for j in range(N):
            if coins[j] > i or dp[i - coins[j]] == float('inf'):
                continue
            dp[i] = min(dp[i], dp[i - coins[j]] + 1)

    # If it is possible to make sum = X, return dp[X]
    if dp[X] != float('inf'):
        return dp[X]

    # If it is impossible to make sum = X, return -1
    return -1

# Sample Input
N = 3
X = 6
coins = [1, 2, 3]

print(solve(N, X, coins))

C#

using System;

public class MinimumCoins
{
    // Function to find the minimum number of coins to make sum X
    public static int Solve(int N, int X, int[] coins)
    {
        // dp[] array such that dp[i] stores the minimum number
        // of coins to make sum = i
        int[] dp = new int[X + 1];
        Array.Fill(dp, int.MaxValue);
        dp[0] = 0;

        // Iterate over all possible sums from 1 to X
        for (int i = 1; i <= X; i++)
        {
            // Iterate over all coins
            for (int j = 0; j < N; j++)
            {
                if (coins[j] > i || dp[i - coins[j]] == int.MaxValue)
                    continue;
                dp[i] = Math.Min(dp[i], dp[i - coins[j]] + 1);
            }
        }

        // If it is possible to make sum = X, return dp[X]
        if (dp[X] != int.MaxValue)
            return dp[X];

        // If it is impossible to make sum = X, return -1
        return -1;
    }

    public static void Main(string[] args)
    {
        // Sample Input
        int N = 3;
        int X = 6;
        int[] coins = new int[] { 1, 2, 3 };

        Console.WriteLine(Solve(N, X, coins));
    }
}

JavaScript

// Function to find the minimum number of coins to make sum X
function solve(N, X, coins) {
    // dp[] array such that dp[i] stores the minimum number
    // of coins to make sum = i
    let dp = new Array(X + 1).fill(Number.MAX_SAFE_INTEGER);
    dp[0] = 0;

    // Iterate over all possible sums from 1 to X
    for (let i = 1; i <= X; i++) {
        // Iterate over all coins
        for (let j = 0; j < N; j++) {
            if (coins[j] > i || dp[i - coins[j]] === Number.MAX_SAFE_INTEGER)
                continue;
            dp[i] = Math.min(dp[i], dp[i - coins[j]] + 1);
        }
    }
    // If it is possible to make sum = X, return dp[X]
    if (dp[X] !== Number.MAX_SAFE_INTEGER)
        return dp[X];

    // If it is impossible to make sum = X, return -1
    return -1;
}

// Sample Input
let N = 3, X = 6;
let coins = [1, 2, 3];

console.log(solve(N, X, coins));

Output

2

Time Complexity: O(N * X), where N is the number of coins and X is the sum of coins we want to make.
Auxiliary Space: O(X)