#include<bits/stdc++.h> using namespace std; int a[1000007]; int mids(int s[],int high,int low,int m) { int mid=(high+low)/2; if(s[high]<m) { cout<<-1; } else if(s[mid]==m) { cout<<mid; } else if(m<s[mid]) { return mids(s,mid-1,low,m); } else if(m>s[mid]) { return mids(s,high,mid+1,m); } } int mian() { int n,m; cin>>n>>m; for(int i=1;i<=n;i++) cin>>a[i]; mids(a,n,1,m); return 0; }

using namespace std;
int a[1000000];
bool flag=false;
void f(int c,int d)
{
	if(c>0){
		if(a[c]==d) flag=true;
		else return f(c-1,d);
		if(flag) cout<<c;
	}
	else{
		cout<<"-1";
	}
}
int main()
{
	int n,m;
	cin>>n>>m;
	for(int i=1;i<=n;i++){
		cin>>a[i];
	}
	f(n,m);
	return 0;
}

555

二分模版题。

#include<bits/stdc++.h>
using namespace std;
const int N=1e7+5;
int a[N],n,k;
int bs(int l,int r,int x){
	if(l>r)	return -1;
	int mid=(l+r)>>1;
	if(a[mid]==x)	return mid;
	else if(x>a[mid])	return bs(mid+1,r,x);//向右找
	else return bs(l,mid-1,x);//向左找
}
int main(){
	cin>>n>>k;
	for(int i=1;i<=n;i++)	cin>>a[i];
	cout<<bs(1,n+1,k);
}

诏曰

#include<bits/stdc++.h>
using namespace std;
int nums[100000000],n,m;
int BinSearch(int r[],int low,int high,int num){
	int mid;
	if(low>high) return -1;
	else{
		mid=(low+high)/2;
		if(num>r[mid]) return BinSearch(r,mid+1,high,num);
		else if(num<r[mid]) return BinSearch(r,low,mid-1,num);
		else return mid;
	}
}
int main(){
	cin>>n>>m; for(int i=1;i<=n;i++) cin>>nums[i];
	cout<<BinSearch(nums,1,n,m);
	return 0;
}