arr 是货币数组,其中的值都是正数。再给定一个正数 aim
每个值都认为是一张货币,认为值相同的货币没有任何不同,返回组成 aim 的方法数 例如:arr={1,2,1,1,2,1,2}, aim=4
方法:1+1+1+1
、1+1+2
、2+2
一个就三种方法,返回 3
暴力递归
public static class Info {
public int[] coins;
public int[] zhangs;
public Info(int[] c, int[] z) {
coins = c;
zhangs = z;
}
}
public static Info getInfo(int[] arr) {
HashMap<Integer, Integer> counts = new HashMap<>();
for (int value : arr) {
if (!counts.containsKey(value))
counts.put(value, 1);
else
counts.put(value, counts.get(value) + 1);
}
int N = counts.size();
int[] coins = new int[N];
int[] zhangs = new int[N];
int index = 0;
for (Entry<Integer, Integer> entry : counts.entrySet()) {
coins[index] = entry.getKey();
zhangs[index++] = entry.getValue();
}
return new Info(coins, zhangs);
}
public static int coinsWay(int[] arr, int aim) {
if (arr == null || arr.length == 0 || aim < 0) {
return 0;
}
Info info = getInfo(arr);
return process(info.coins, info.zhangs, 0, aim);
}
DP(有优化空间)
public static int dp1(int[] arr, int aim) {
if (arr == null || arr.length == 0 || aim < 0) {
return 0;
}
Info info = getInfo(arr);
int[] coins = info.coins;
int[] zhangs = info.zhangs;
int N = coins.length;
int[][] dp = new int[N + 1][aim + 1];
dp[N][0] = 1;
for (int index = N - 1; index >= 0; index--) {
for (int rest = 0; rest <= aim; rest++) {
int ways = 0;
for (int zhang = 0; zhang * coins[index] <= rest && zhang <= zhangs[index]; zhang++) {
ways += dp[index + 1][rest - (zhang * coins[index])];
}
dp[index][rest] = ways;
}
}
return dp[0][aim];
}
DP(优化决策)
public static int dp2(int[] arr, int aim) {
if (arr == null || arr.length == 0 || aim < 0) {
return 0;
}
Info info = getInfo(arr);
int[] coins = info.coins;
int[] zhangs = info.zhangs;
int N = coins.length;
int[][] dp = new int[N + 1][aim + 1];
dp[N][0] = 1;
for (int index = N - 1; index >= 0; index--) {
for (int rest = 0; rest <= aim; rest++) {
dp[index][rest] = dp[index + 1][rest];
if (rest - coins[index] >= 0) {
dp[index][rest] += dp[index][rest - coins[index]];
}
if (rest - coins[index] * (zhangs[index] + 1) >= 0) {
dp[index][rest] -= dp[index + 1][rest - coins[index] * (zhangs[index] + 1)];
}
}
}
return dp[0][aim];
}