牛客练习赛142-C.战前准备 · znzryb

## 思路讲解

差分做法，只要用差分把所有坏区间都标记出来，剩下的就是不可行的。

```cpp
	FOR(i,0,M-2){
		for(auto &p:pos[i+1]){
			auto it=lower_bound(all(pos[i]),p);
			if(it==pos[i].begin()){
				diff[1]+=1;
				// p+1是可行的（至少从这个p看来）
				diff[p+1]-=1;
			}else{
				it--;ll idx=*it;
				diff[idx+1]+=1;
				// p+1是可行的（至少从这个p看来）
				diff[p+1]-=1;
			}
		}
	}
```

## AC代码

[https://ac.nowcoder.com/acm/contest/view-submission?submissionId=78135312](https://ac.nowcoder.com/acm/contest/view-submission?submissionId=78135312)



::: 源代码

```cpp
// Problem: 战前准备
// Contest: NowCoder
// URL: https://ac.nowcoder.com/acm/contest/112575/C
// Memory Limit: 512 MB
// Time Limit: 2000 ms
// 
// Powered by CP Editor (https://cpeditor.org)

#include <bits/stdc++.h>
#define all(vec) vec.begin(),vec.end()
#define pb push_back
#define SZ(a) ((int) a.size())
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define ROF(i, a, b) for (int i = (a); i >= (b); --i)
#define debug(var) cerr << #var <<":"<<var<<"\n";
#define lson(var) (var<<1)
#define rson(var) ((var<<1)+1)

using namespace std;

using  ll = long long;using ull = unsigned long long;
using DB=double;using LD=long double;
// using i128 = __int128;
using pdd = pair<DB,DB>;using plb = pair<ll,bool>;
using pll = pair<ll,ll>;
using arr3 = array<ll,3> ;using arr2 = array<ll,2>;
constexpr ll MAXN=static_cast<ll>(2e5)+10,INF=static_cast<ll>(1e17)+9; // 1e18+9也是素数
constexpr ll MAXM=(ll)1e6+10;constexpr ll MAXV=(ll)1e6+10;
constexpr ll mod=static_cast<ll>(1e9)+7;
constexpr double eps=1e-8;const double pi=acos(-1.0);

ll N,M,Q,X,K,T,lT,A[MAXN];
/*

*/
vector<ll> pos[MAXN];
ll diff[MAXN],Sum[MAXN];
inline void init(){

}
inline void solve(){
	cin>>N>>M;
	init();
	FOR(i,1,N){
		cin>>A[i];
		pos[A[i]].pb(i);
	}
	if(M==1){
		cout<<N<<"\n";return;
	}
	FOR(i,0,M-2){
		ll las=0;
		for(auto &p:pos[i+1]){
			auto it=lower_bound(all(pos[i]),p);
			if(it==pos[i].begin()){
				diff[1]+=1;
				// p+1是可行的（至少从这个p看来）
				diff[p+1]-=1;
			}else{
				it--;ll idx=*it;
				diff[idx+1]+=1;
				// p+1是可行的（至少从这个p看来）
				diff[p+1]-=1;
			}
		}
	}
	FOR(i,1,N){
		Sum[i]=Sum[i-1]+diff[i];
	}
	ll ans=0;
	FOR(i,1,N){
		if(Sum[i]==0){
			++ans;
		}
	}
	cout<<ans<<"\n";
}

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	
	// cin>>lT;
	// while(lT--)
		solve();
	return 0;
}
/*
AC
https://ac.nowcoder.com/acm/contest/view-submission?submissionId=78135312
*/
```


:::


## 心路历程（WA，TLE，MLE……）