77 lines
2.2 KiB
Markdown
Executable File
77 lines
2.2 KiB
Markdown
Executable File
# algorithm_2024
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algorithm_2024
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## 错题本
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### Luogu某题
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#### 数组越界导致变量异常更改
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### [OJ4980:拯救行动](http://noi.openjudge.cn/ch0205/4980/)
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#### 未考虑无答案（特殊情况）时输出
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#### 优先队列是从大到小排序，重载运算符时需反向或者std::greater
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```cpp
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for(ll i{0};i<4;i++){
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const Point next {status.now.x+to_next[i][0],status.now.y+to_next[i][1]};
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if(vis[next.x][next.y])continue;
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const auto nextchar = [&next]()->char{return map[next.x][next.y];};
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ll cost {1};
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if(next.x>h || next.x<=0 || next.y > w || next.y<=0
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|| nextchar()=='#')continue;
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if(nextchar()=='x')cost++; // 因为这里有可能会遇到士兵，会改变最优解顺序，要使用priority_queue
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const Status next_status {next,status.step+cost};
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vis[next_status.now.x][next_status.now.y] = true;
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q.push(next_status);
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}
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```
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```cpp
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struct Status{
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Point now;
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ll step;
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bool operator<(const Status &that)const noexcept{
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return this->step > that.step;
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}
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};
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std::priority_queue<Status> q;
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```
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### [P1330](https://www.luogu.com.cn/problem/P1330)
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#### BFS时注意初始化一开始的去重数组
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```cpp
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void bfs(){
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for(ll i{1};i<=n;i++){
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color_sum[1]=color_sum[2]=0;
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if(vis[i])continue;
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q.push(i);
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set_color(i, 1);
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vis[i]=true; // 注意初始化错误
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while(!q.empty()){
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```
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### [P3957](https://www.luogu.com.cn/problem/P3957)
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#### 初始状态依赖已走过的部分时注意起始点状态
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```cpp
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for(ll coin{0};coin<=(points[n].posit-d);++coin){
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for(ll i{0};i<max_n;i++)dp[i]=ll_min;
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dp[0]=0; // 注意第0个点是能到达的reachable
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```
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#### 非最优解时注意骗分卡时间
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```cpp
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const ll max_coin{(ll)1e5+5};//d+g = x[n] -> g = x[n]-d我的推导是这样的，但是错了，必须将max_coin设置为1e5+5也就是s[i]最大值，注意超时问题，可以自己生成样例测试
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ll l{0},r{max_coin},ans{ll_max};
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while(l<=r){
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ll mid{(l+r)/2};
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const bool check_ret{check(mid)};
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if(check_ret){
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ans = mid;
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r=mid-1;
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}else{
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l=mid+1;
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}
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}
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``` |