Codeforces 474D - Flowers (0 - 1背包)
题意
小明要吃花。
他只能吃k朵连着的白色花，不然就不吃，问花有a~b朵的时候有几种情况
思路
就是一个简单的背包
$dp[i] = dp[i - 1] + dp[i - k]$
有i朵花的时候，要么增加一朵红花，这时候是dp[i - 1]种情况，也可以增加k朵白花，dp[i - k]种情况。
比赛的时候我竟然写了排列组合，妥妥的TLE。
代码
#include <cstdio>
#include <stack>
#include <set>
#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <functional>
#include <cstring>
#include <algorithm>
#include <cctype>
#include <ctime>
#include <cstdlib>
#include <fstream>
#include <string>
#include <sstream>
#include <map>
#include <cmath>
#define LL long long
#define SZ(x) (int)x.size()
#define Lowbit(x) ((x) & (-x))
#define MP(a, b) make_pair(a, b)
#define MS(arr, num) memset(arr, num, sizeof(arr))
#define PB push_back
#define F first
#define S second
#define ROP freopen("input.txt", "r", stdin);
#define MID(a, b) (a + ((b - a) >> 1))
#define LC rt << 1, l, mid
#define RC rt << 1|1, mid + 1, r
#define LRT rt << 1
#define RRT rt << 1|1
#define BitCount(x) __builtin_popcount(x)
const double PI = acos(-1.0);
const int INF = 0x3f3f3f3f;
using namespace std;
const int MAXN = 1e5 + 10;
const int MOD = 1e9 + 7;
typedef pair<int, int> pii;
typedef vector<int>::iterator viti;
typedef vector<pii>::iterator vitii;
LL dp[MAXN];
int main()
{
//ROP;
ios::sync_with_stdio(0);
int n, k, i, j;
cin >> n >> k;
dp[0] = 1;
for (i = 1; i < MAXN; i++) dp[i] = (dp[i - 1] + (i >= k ? dp[i - k] : 0)) % MOD;
for (i = 2; i < MAXN; i++) dp[i] = (dp[i - 1] + dp[i]) % MOD;
dp[0] = 0;
while (n--)
{
int a, b;
cin >> a >> b;
cout << (dp[b] - dp[a - 1] + MOD) % MOD << endl;
}
return 0;
}