点击打开链接 挡板法求解 参考博客点击打开链接   #include <bits/stdc++.h>using namespace std;#define ll long long#define M 1000000007ll quickpow(ll a,ll b){ll ans;ans=1;while(b>0){if(b%2==1){ans=(ans*a)%M;}a=(a*a)%M;