背包问题、线性DP、区间DP、状态压缩DP、树型DP、计数类DP、数位统计DP、记忆化搜索(实现方式)
从集合的角度来思考,一般来说从两个角度来考虑,即状态表示和状态计算。状态表示,思考需要用几维来表示状态,状态计算则考虑如何将状态计算出来。状态表示从两个角度来考虑,一是集合的含义,二是属性:一般取 MAX / MIN / 数量
对代码或者计算方程做等价变形
N件物品、容量为V的背包,每个物品有两个属性vi和wi,每件物品最多用一次,挑一些物品使得总体积<=V,使价值达到最大
AcWing 2. 01背包问题
// 朴素版本
#include
#include
#include
using namespace std;
const int N = 1005;
int n, m;
int v[N], w[N], f[N][N];
int main( )
{
cin >> n >> m;
for (int i = 1; i <= n; i ++)
cin >> v[i] >> w[i];
for (int i = 1; i <= n; i ++)
for (int j = 1; j <= m; j ++)
{
if (v[i] <= j)
f[i][j] = max(f[i - 1][j], f[i - 1][j - v[i]] + w[i]);
else
f[i][j] = f[i - 1][j];
}
cout << f[n][m] << endl;
return 0;
}
// 优化版本
#include
#include
#include
using namespace std;
const int N = 1005;
int n, m;
int v[N], w[N], f[N];
int main( )
{
cin >> n >> m;
for (int i = 1; i <= n; i ++)
cin >> v[i] >> w[i];
for (int i = 1; i <= n; i ++)
for (int j = m; j >= v[i]; j --)
f[j] = max(f[j], f[j - v[i]] + w[i]);
cout << f[m] << endl;
return 0;
}
每件物品有无限个
AcWing 3. 完全背包问题
// 朴素版本,最坏时间复杂度为O(N³)
#include
#include
#include
using namespace std;
const int N = 1010;
int n, m;
int v[N], w[N], f[N][N];
int main( )
{
cin >> n >> m;
for (int i = 1; i <= n; i ++) cin >> v[i] >> w[i];
for (int i = 1; i <= n; i ++)
for (int j = 0; j <= m; j ++)
for (int k = 0; k * v[i] <= j; k ++)
{
f[i][j] = max(f[i][j], f[i - 1][j - k * v[i]] + k * w[i]);
}
cout << f[n][m] << endl;
return 0;
}
// 二维优化版本
#include
#include
#include
using namespace std;
const int N = 1010;
int n, m;
int v[N], w[N], f[N][N];
int main( )
{
cin >> n >> m;
for (int i = 1; i <= n; i ++) cin >> v[i] >> w[i];
for (int i = 1; i <= n; i ++)
for (int j = 0; j <= m; j ++)
{
f[i][j] = f[i - 1][j];
if (j >= v[i]) f[i][j] = max(f[i][j], f[i][j - v[i]] + w[i]);
}
cout << f[n][m] << endl;
return 0;
}
// 究极优化一维版本
#include
#include
#include
using namespace std;
const int N = 1010;
int n, m;
int v[N], w[N], f[N];
int main( )
{
cin >> n >> m;
for (int i = 1; i <= n; i ++) cin >> v[i] >> w[i];
for (int i = 1; i <= n; i ++)
for (int j = v[i]; j <= m; j ++)
f[j] = max(f[j], f[j - v[i]] + w[i]);
cout << f[m] << endl;
return 0;
}
每种物品有 si 个
Acwing 4. 多重背包 I
Acwing 5. 多重背包 II
// 朴素做法
#include
using namespace std;
const int N = 105;
int n, m;
int v[N], w[N], s[N];
int f[N][N];
int main()
{
cin >> n >> m;
for (int i = 1; i <= n; i ++) cin >> v[i] >> w[i] >> s[i];
for (int i = 1; i <= n; i ++)
for (int j = 0; j <= m; j ++)
for (int k = 0; k <= s[i] && k * v[i] <= j; k ++)
f[i][j] = max(f[i][j], f[i - 1][j - k * v[i]] + w[i] * k);
cout << f[n][m] << endl;
return 0;
}
// 0 < N < 2000 二进制优化做法
#include
using namespace std;
const int N = 25000;
int n, m;
int v[N], w[N], s[N];
int f[N];
int main()
{
cin >> n >> m;
int cnt = 0;
for (int i = 1; i <= n; i ++) // 分组
{
int a, b, s;
cin >> a >> b >> s;
int k = 1;
while (k <= s)
{
cnt ++;
v[cnt] = a * k;
w[cnt] = b * k;
s -= k;
k *= 2;
}
if (s > 0)
{
cnt ++;
v[cnt] = s * a;
w[cnt] = s * b;
}
}
n = cnt;
for (int i = 1; i <= n; i ++)
for (int j = m; j >= v[i]; j --)
f[j] = max(f[j], f[j - v[i]] + w[i]);
cout << f[m] << endl;
return 0;
}
有N组物品,每组物品里有若干个,每一组里至多只能选一个物品(互斥)
Acwing 9. 分组背包问题
#include
using namespace std;
const int N = 110;
int n, m;
int v[N][N], w[N][N], s[N];
int f[N];
int main()
{
cin >> n >> m;
for (int i = 1; i <= n; i ++)
{
cin >> s[i]; // 第i组物品数
for (int j = 1; j <= s[i]; j ++) cin >> v[i][j] >> w[i][j]; // 第i组第j件物品的v, w
}
for (int i = 1; i <= n; i ++)
for (int j = m; j >= 0; j --)
for (int k = 0; k <= s[i]; k ++)
if (v[i][k] <= j) // 第i组第k件物品容的下
f[j] = max(f[j], f[j - v[i][k]] + w[i][k]);
cout << f[m] << endl;
return 0;
}
所有递推顺序有一个模糊的线性顺序
Acwing 898. 数字三角形
#include
using namespace std;
const int N = 550, INF = 1e9;
int n;
int a[N][N];
int f[N][N];
int main()
{
cin >> n;
for (int i = 1; i <= n; i ++)
for (int j = 1; j <= i; j ++)
cin >> a[i][j];
// 初始化
for (int i = 0; i <= n; i ++)
for (int j = 0; j <= i + 1; j ++)
f[i][j] = -INF;
f[1][1] = a[1][1];
// 动态规划
for (int i = 2; i <= n; i ++)
for (int j = 1; j <= i; j ++)
f[i][j] = max(f[i - 1][j - 1] + a[i][j], f[i - 1][j] + a[i][j]);
int res = -INF;
for (int i = 1; i <= n; i ++) res = max(res, f[n][i]);
cout << res << endl;
return 0;
}
Acwing 895. 最长上升子序列
#include
using namespace std;
const int N = 1005;
int n;
int a[N], f[N];
int main()
{
cin >> n;
for (int i = 1; i <= n; i ++) cin >> a[i];
for (int i = 1; i <= n; i ++)
{
f[i] = 1;
for (int j = 1; j <= i; j ++)
if (a[j] < a[i])
f[i] = max(f[i], f[j] + 1);
}
int res = 0;
for (int i = 1; i <= n; i ++) res = max(res, f[i]);
cout << res << endl;
return 0;
}
AcWing 897. 最长公共子序列
#include
using namespace std;
const int N = 1050;
int n, m;
char a[N], b[N];
int f[N][N];
int main()
{
cin >> n >> m;
cin >> a + 1 >> b + 1;
for (int i = 1; i <= n; i ++)
for (int j = 1; j <= m; j ++)
{
f[i][j] = max(f[i - 1][j], f[i][j - 1]);
if (a[i] == b[j])
f[i][j] = max(f[i][j], f[i - 1][j - 1] + 1);
}
cout << f[n][m] << endl;
return 0;
}
AcWing 282. 石子合并
#include
using namespace std;
const int N = 305;
int n;
int s[N];
int f[N][N];
int main()
{
cin >> n;
for (int i = 1; i <= n; i ++) cin >> s[i];
for (int i = 1; i <= n; i ++) s[i] += s[i - 1];
for (int len = 2; len <= n; len ++) // 枚举区间长度
for (int i = 1; i + len - 1 <= n; i ++) // 枚举左右端点
{
int l = i, r = i + len - 1;
f[l][r] = 1e9;
for (int k = l; k <= r - 1; k ++) // 枚举分界线
f[l][r] = min(f[l][r], f[l][k] + f[k + 1][r] + s[r] - s[l - 1]);
}
cout << f[1][n] << endl;
return 0;
}
AcWing 900. 整数划分
#include
using namespace std;
const int N = 1050, MOD = 1e9 + 7;
int n;
int f[N];
int main()
{
cin >> n;
f[0] = 1;
for (int i = 1; i <= n; i ++)
for (int j = i; j <= n; j ++)
f[j] = (f[j] + f[j - i]) % MOD;
cout << f[n] << endl;
return 0;
}
#include
using namespace std;
const int N = 1010, MOD = 1e9 + 7;
int n;
int f[N][N];
int main()
{
cin >> n;
f[0][0] = 1;
for (int i = 1; i <= n; i ++)
for (int j = 1; j <= i; j ++)
f[i][j] = (f[i - 1][j - 1] + f[i - j][j]) % MOD;
int res = 0;
for (int i = 1; i <= n; i ++) res = (res + f[n][i]) % MOD;
cout << res << endl;
return 0;
}
AcWing 338. 计数问题
#include
#include
#include
using namespace std;
const int N = 10;
/*
001~abc-1, 999
abc
1. num[i] < x, 0
2. num[i] == x, 0~efg
3. num[i] > x, 0~999
*/
int get(vector<int> num, int l, int r)
{
int res = 0;
for (int i = l; i >= r; i -- ) res = res * 10 + num[i];
return res;
}
int power10(int x)
{
int res = 1;
while (x -- ) res *= 10;
return res;
}
int count(int n, int x)
{
if (!n) return 0;
vector<int> num;
while (n)
{
num.push_back(n % 10);
n /= 10;
}
n = num.size();
int res = 0;
for (int i = n - 1 - !x; i >= 0; i -- )
{
if (i < n - 1)
{
res += get(num, n - 1, i + 1) * power10(i);
if (!x) res -= power10(i);
}
if (num[i] == x) res += get(num, i - 1, 0) + 1;
else if (num[i] > x) res += power10(i);
}
return res;
}
int main()
{
int a, b;
while (cin >> a >> b , a)
{
if (a > b) swap(a, b);
for (int i = 0; i <= 9; i ++ )
cout << count(b, i) - count(a - 1, i) << ' ';
cout << endl;
}
return 0;
}
AcWing 91. 最短Hamilton路径
#include
#include
#include
using namespace std;
const int N = 20, M = 1 << N;
int w[N][N];
int f[M][N];
int n;
int main()
{
cin >> n;
for (int i = 0; i < n; i ++)
for (int j = 0; j < n; j ++)
cin >> w[i][j];
memset(f, 0x3f, sizeof f);
f[1][0] = 0; // 表示从 0 到 0 且经过的所有点为{ 0 }的方案
for (int i = 1; i < 1 << n; i ++) // 二进制表示所有状态,共有2^n 个状态
for (int j = 0; j < n; j ++) // 枚举每一个点
if (i >> j & 1) // 如果当前方案经过 j
for (int k = 0; k < n; k ++) // 枚举倒数第二个点
if (i >> k & 1) // 若当前方案经过k点
f[i][j] = min(f[i][j], f[i - (1 << j)][k] + w[k][j]); // 更新方案
cout << f[(1 << n) - 1][n - 1] << endl;
return 0;
}
AcWing 285. 没有上司的舞会
#include
#include
#include
using namespace std;
const int N = 6010;
int h[N], e[N], ne[N], idx;
int happy[N];
int f[N][2];
int n;
bool st[N]; // 判断是否为root
void add(int a, int b) // a -> b
{
e[idx] = b, ne[idx] = h[a], h[a] = idx ++;
}
void dfs(int u)
{
f[u][1] = happy[u];
for (int i = h[u]; i != -1; i = ne[i])
{
int j = e[i]; // j 为 u 的所有子节点
dfs(j);
f[u][0] += max(f[j][0], f[j][1]);
f[u][1] += f[j][0];
}
}
int main()
{
cin >> n; // n 个点
for (int i = 1; i <= n; i ++) cin >> happy[i]; // 输入高兴值
memset(h, -1, sizeof h); // 初始化所有头节点
for (int i = 0; i < n - 1; i ++)
{
int a, b;
cin >> a >> b; // b 是 a 的父节点
add(b, a);
st[a] = true;
}
// 找根节点
int root = 1;
while (st[root]) root ++;
// 树形DP
dfs(root);
// 输出答案
cout << max(f[root][0], f[root][1]) << endl;
return 0;
}
Acwing 901. 滑雪
#include
#include
#include
using namespace std;
const int N = 305;
int w[N][N];
int f[N][N];
int n, m;
int dx[4] = {-1, 0, 1, 0}, dy[4] = {0, 1, 0, -1};
bool check(int x, int y)
{
return x >= 1 && x <= n && y >= 1 && y <= m;
}
int dp(int x, int y)
{
int &v = f[x][y];
if (v != -1) return v;
v = 1;
for (int i = 0; i < 4; i ++)
{
int a = x + dx[i], b = y + dy[i];
if (check(a, b) && w[a][b] < w[x][y]) // 不越界,且下一格子小于当前格子
v = max(v, dp(a, b) + 1);
}
return v;
}
int main()
{
cin >> n >> m;
for (int i = 1; i <= n; i ++)
for (int j = 1; j <= m; j ++)
cin >> w[i][j];
memset(f, -1, sizeof f);
int res = 0;
for (int i = 1; i <= n; i ++)
for (int j = 1; j <= m; j ++)
res = max(res, dp(i, j));
cout << res << endl;
return 0;
}