feat: 添加P7960和P7961题解并优化P5835代码

添加两道新题目的解决方案：
1. P7960.cpp实现数字7相关筛选算法
2. P7961.cpp实现组合数学问题的DFS解法

优化P5835.cpp的逆序对计算：
- 修改合并排序的比较条件
- 改进权重分配逻辑
- 增加代码可读性和注释

src/10/22/P5835.cpp

@@ -4,6 +4,7 @@
 #include <iostream>
 #include <istream>
 #include <map>
+#include <string>
 #include <unordered_set>
 #include <vector>
 using ll = int64_t;
@@ -29,7 +30,7 @@ static inline ll merge(std::vector<ll>&num,std::vector<ll>&tmp,ll l,ll mid,ll r)
     ll cnt=0;
     ll i=l,j=mid+1,k=l;
     while(i<=mid&&j<=r){
-        if(num[i]<=num[j]){
+        if(num[i]<num[j]){
             tmp[k++]=num[i++];
         }else{
             cnt+= mid-i+1;
@@ -77,9 +78,14 @@ int main(){
     }
     std::sort(dis+1,dis+1+n);
     ll outweight=0;
-    ll pout=0;
+    ll pout=0,lout=1,rout=n;
     for(ll i=1;i<=n;i++){
-        outweight+=c[dis[i].idx].w;
+        if(c[dis[i].idx].d==1){
+            outweight+=c[rout--].w;
+        }else{
+            outweight+=c[lout++].w;
+        }
+        // outweight+=c[dis[i].idx].w;
         // printf("%lld is out\n",c[dis[i].idx].x);
         pout=dis[i].d;
         vis.emplace(dis[i].idx);
@@ -95,8 +101,9 @@ int main(){
     // std::cout<<"\n";
     after.push_back(0);
     for(ll i=1;i<=n;i++){
-        after.push_back(c[i].x+c[i].d*pout);
+        ll na=c[i].x+c[i].d*pout;
+        after.push_back(na);
     }
-    
-    std::cout<<mergesort(after, tmp, 1, n)<<"\n";
+    const ll ans = mergesort(after, tmp, 1, n);
+    std::cout<<ans<<"\n";
 }

src/10/23/P7960.cpp

@@ -0,0 +1,44 @@
+#include <bitset>
+#include <cstdint>
+#include <iostream>
+#include <istream>
+#include <string>
+using ll = int64_t;
+
+const ll maxn = 1e7+7;
+std::bitset<maxn> isnot;
+ll nxtpos[maxn];
+
+int main(){
+    std::iostream::sync_with_stdio(false);
+    std::cin.tie(nullptr);
+
+    for(ll i=1;i<maxn;i++){
+        const std::string str = std::to_string(i);
+        for(char c:str){
+            if(c=='7'){
+                for(ll j=1;j*i<maxn;j++){
+                    isnot[i*j]=true;
+                }
+                break;
+            }
+        }
+    }
+    for(ll i=maxn-1;i>=1;i--){
+        static ll last = maxn;
+        last = (isnot[i]?last:i);
+        nxtpos[i]=last;
+    }
+    ll t;
+    std::cin>>t;
+    while(t--){
+        ll x;
+        std::cin>>x;
+        if(isnot[x]){
+            std::cout<<"-1\n";
+            continue;
+        }
+        x++;
+        std::cout<<nxtpos[x]<<"\n";
+    }
+}

src/10/23/P7961.cpp

@@ -0,0 +1,52 @@
+#include <cstdint>
+#include <cstdio>
+#include <iostream>
+#include <istream>
+using ll = int64_t;
+using int128=__int128;
+const ll maxn = 107,p=998244353;
+ll n,m,k,v[maxn],sa[maxn],ans;
+static inline ll popcount(int128 n){
+    ll ans=0;
+    while(n){
+        ans++;
+        n-=(n&-n);
+    }
+    return ans;
+}
+static inline void getans(int128 add){
+    // printf("getans %lld popcount=%lld\n",add,popcount(add));
+    if(popcount(add)>k){
+        // printf("popcount >k\n");
+        return;
+    }
+    ll nans=1;
+    for(ll i=1;i<=n;i++){
+        nans=nans*v[sa[i]]%p;
+    }
+    ans=(ans+nans)%p;
+}
+
+static inline void dfs(ll idx,int128 add){
+    // printf("dfs %lld %lld\n",idx, add);
+    if(idx>n){
+        getans(add);
+        return;
+    }
+    for(ll i=0;i<=m;i++){
+        sa[idx]=i;
+        dfs(idx+1,add+(1ll<<i));
+    }
+}
+
+int main(){
+    std::iostream::sync_with_stdio(false);
+    std::cin.tie(nullptr);
+
+    std::cin>>n>>m>>k;
+    for(ll i=0;i<=m;i++){
+        std::cin>>v[i];
+    }
+    dfs(1, 0);
+    std::cout<<ans<<"\n";
+}