README_EN.md

08.11. Coin

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Description

Given an infinite number of quarters (25 cents), dimes (10 cents), nickels (5 cents), and pennies (1 cent), write code to calculate the number of ways of representing n cents. (The result may be large, so you should return it modulo 1000000007)

Example1:

 Input: n = 5
 Output: 2
 Explanation: There are two ways:
5=5
5=1+1+1+1+1

Example2:

 Input: n = 10
 Output: 4
 Explanation: There are four ways:
10=10
10=5+5
10=5+1+1+1+1+1
10=1+1+1+1+1+1+1+1+1+1

Notes:

You can assume:

• 0 <= n <= 1000000

Solutions

Solution 1: Dynamic Programming

We define $f[i][j]$ as the number of ways to make up the total amount $j$ using only the first $i$ types of coins. Initially, $f[0][0]=1$, and the rest of the elements are $0$. The answer is $f[4][n]$.

Considering $f[i][j]$, we can enumerate the number of the $i$-th type of coin used, $k$, where $0 \leq k \leq j / c_i$, then $f[i][j]$ is equal to the sum of all $f[i−1][j−k \times c_i]$. Since the number of coins is infinite, $k$ can start from $0$. That is, the state transition equation is as follows:

$$ f[i][j] = f[i - 1][j] + f[i - 1][j - c_i] + \cdots + f[i - 1][j - k \times c_i] $$

Let $j = j - c_i$, then the above state transition equation can be written as:

$$ f[i][j - c_i] = f[i - 1][j - c_i] + f[i - 1][j - 2 \times c_i] + \cdots + f[i - 1][j - k \times c_i] $$

Substitute the second equation into the first equation to get:

$$ f[i][j]= \begin{cases} f[i - 1][j] + f[i][j - c_i], & j \geq c_i \\ f[i - 1][j], & j < c_i \end{cases} $$

The final answer is $f[4][n]$.

The time complexity is $O(C \times n)$, and the space complexity is $O(C \times n)$, where $C$ is the number of types of coins.

We notice that the calculation of $f[i][j]$ is only related to $f[i−1][..]$, so we can remove the first dimension and optimize the space complexity to $O(n)$.

class Solution:
    def waysToChange(self, n: int) -> int:
        mod = 10**9 + 7
        coins = [25, 10, 5, 1]
        f = [[0] * (n + 1) for _ in range(5)]
        f[0][0] = 1
        for i, c in enumerate(coins, 1):
            for j in range(n + 1):
                f[i][j] = f[i - 1][j]
                if j >= c:
                    f[i][j] = (f[i][j] + f[i][j - c]) % mod
        return f[-1][n]

class Solution {
    public int waysToChange(int n) {
        final int mod = (int) 1e9 + 7;
        int[] coins = {25, 10, 5, 1};
        int[][] f = new int[5][n + 1];
        f[0][0] = 1;
        for (int i = 1; i <= 4; ++i) {
            for (int j = 0; j <= n; ++j) {
                f[i][j] = f[i - 1][j];
                if (j >= coins[i - 1]) {
                    f[i][j] = (f[i][j] + f[i][j - coins[i - 1]]) % mod;
                }
            }
        }
        return f[4][n];
    }
}

class Solution {
public:
    int waysToChange(int n) {
        const int mod = 1e9 + 7;
        vector<int> coins = {25, 10, 5, 1};
        int f[5][n + 1];
        memset(f, 0, sizeof(f));
        f[0][0] = 1;
        for (int i = 1; i <= 4; ++i) {
            for (int j = 0; j <= n; ++j) {
                f[i][j] = f[i - 1][j];
                if (j >= coins[i - 1]) {
                    f[i][j] = (f[i][j] + f[i][j - coins[i - 1]]) % mod;
                }
            }
        }
        return f[4][n];
    }
};

func waysToChange(n int) int {
	const mod int = 1e9 + 7
	coins := []int{25, 10, 5, 1}
	f := make([][]int, 5)
	for i := range f {
		f[i] = make([]int, n+1)
	}
	f[0][0] = 1
	for i := 1; i <= 4; i++ {
		for j := 0; j <= n; j++ {
			f[i][j] = f[i-1][j]
			if j >= coins[i-1] {
				f[i][j] = (f[i][j] + f[i][j-coins[i-1]]) % mod
			}
		}
	}
	return f[4][n]
}

function waysToChange(n: number): number {
    const mod = 10 ** 9 + 7;
    const coins: number[] = [25, 10, 5, 1];
    const f: number[][] = Array(5)
        .fill(0)
        .map(() => Array(n + 1).fill(0));
    f[0][0] = 1;
    for (let i = 1; i <= 4; ++i) {
        for (let j = 0; j <= n; ++j) {
            f[i][j] = f[i - 1][j];
            if (j >= coins[i - 1]) {
                f[i][j] = (f[i][j] + f[i][j - coins[i - 1]]) % mod;
            }
        }
    }
    return f[4][n];
}

Solution 2

class Solution:
    def waysToChange(self, n: int) -> int:
        mod = 10**9 + 7
        coins = [25, 10, 5, 1]
        f = [1] + [0] * n
        for c in coins:
            for j in range(c, n + 1):
                f[j] = (f[j] + f[j - c]) % mod
        return f[n]

class Solution {
    public int waysToChange(int n) {
        final int mod = (int) 1e9 + 7;
        int[] coins = {25, 10, 5, 1};
        int[] f = new int[n + 1];
        f[0] = 1;
        for (int c : coins) {
            for (int j = c; j <= n; ++j) {
                f[j] = (f[j] + f[j - c]) % mod;
            }
        }
        return f[n];
    }
}

class Solution {
public:
    int waysToChange(int n) {
        const int mod = 1e9 + 7;
        vector<int> coins = {25, 10, 5, 1};
        int f[n + 1];
        memset(f, 0, sizeof(f));
        f[0] = 1;
        for (int c : coins) {
            for (int j = c; j <= n; ++j) {
                f[j] = (f[j] + f[j - c]) % mod;
            }
        }
        return f[n];
    }
};

func waysToChange(n int) int {
	const mod int = 1e9 + 7
	coins := []int{25, 10, 5, 1}
	f := make([]int, n+1)
	f[0] = 1
	for _, c := range coins {
		for j := c; j <= n; j++ {
			f[j] = (f[j] + f[j-c]) % mod
		}
	}
	return f[n]
}

function waysToChange(n: number): number {
    const mod = 10 ** 9 + 7;
    const coins: number[] = [25, 10, 5, 1];
    const f: number[] = new Array(n + 1).fill(0);
    f[0] = 1;
    for (const c of coins) {
        for (let i = c; i <= n; ++i) {
            f[i] = (f[i] + f[i - c]) % mod;
        }
    }
    return f[n];
}