[HAOI2012]音量调节

嘟嘟嘟

 

这道题只要状态一想出来，这题就做完了。

另 dp[i][j] 表示 i 首歌音量 j 能否达到，则如果dp[i - 1][j] = 1，那么dp[i][j + c[i]] = dp[i][j - c[i]] = 1.然后最后从Max到0反向遍历dp[n][i]即可。

注意这题数组要开2e3，否则因为j + c[i]数组越界造成了一些诡异的错误，导致我第一次交WA了三个点。

 1 #include<cstdio>
 2 #include<iostream>
 3 #include<cmath>
 4 #include<algorithm>
 5 #include<cstring>
 6 #include<cstdlib>
 7 #include<cctype>
 8 #include<vector>
 9 #include<stack>
10 #include<queue>
11 using namespace std;
12 #define enter printf("\n")
13 #define space printf(" ")
14 #define Mem(a) memset(a, 0, sizeof(a))
15 typedef long long ll;
16 typedef double db;
17 const int INF = 0x3f3f3f3f;
18 const int eps = 1e-8;
19 const int maxn = 55;
20 const int max_size = 2e3 + 5;
21 inline ll read()
22 {
23     ll ans = 0;
24     char ch = getchar(), last = ' ';
25     while(!isdigit(ch)) {last = ch; ch = getchar();}
26     while(isdigit(ch)) {ans = ans * 10 + ch - '0'; ch = getchar();}
27     if(last == '-') ans = -ans;
28     return ans;
29 }
30 inline void write(ll x)
31 {
32     if(x < 0) x = -x, putchar('-');
33     if(x >= 10) write(x / 10);
34     putchar(x % 10 + '0');
35 }
36 
37 int n, Beg, Max, a[maxn];
38 bool dp[maxn][max_size];
39 
40 int main()
41 {
42     n = read(); Beg = read(), Max = read();
43     for(int i = 1; i <= n; ++i) a[i] = read();
44     dp[0][Beg] = 1;
45     for(int i = 1; i <= n; ++i)
46         for(int j = 0; j <= Max; ++j)
47             if(dp[i - 1][j]) dp[i][j + a[i]] = dp[i][j - a[i]] = 1;
48     for(int i = Max; i >= 0; --i) if(dp[n][i]) {write(i); enter; return 0;}
49     write(-1); enter;
50     return 0;
51 }

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