[蓝桥杯]真题讲解：子串简写（暴力+二分）

一、视频讲解

视频讲解

二、暴力代码

//暴力代码：
#include
#define int long long
using namespace std;

void solve()
{
	int k; 
	string s;
	char c1, c2;
	cin >> k >> s >> c1 >> c2;
	int ans = 0;
	for(int i = 0; i < s.size(); i ++)
	{
		if(s[i] != c1)
			continue;
		for(int j = i + 1; j < s.size(); j ++)
		{
			int len = j - i + 1;
			if(len >= k && s[j] == c2)
				ans ++;
		}
	}
	cout << ans << endl;
}

signed main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	int t = 1;
	// cin >> t;
	while(t--)
	solve();
}

三、正解代码

//子串简写：二分
#include
#define int long long
using namespace std;

void solve()
{
	int k;string s;
	char c1, c2;
	cin >> k >> s >> c1 >> c2;
	
	vector<int>pc1;//存储c1的位置

	int ans = 0;	
	for(int i = 0; i < s.size(); i ++)
	{
		if(s[i] == c1)
			pc1.push_back(i);
		
		if(s[i] == c2)
		{
		    if(i - k + 1 < 0 || !pc1.size())
		        continue;

		    int l = 0, r = (int)pc1.size() - 1;
		    while(l < r)
		    {
		    	int mid = l + r + 1 >> 1;
		    	if(pc1[mid] <= (i - k + 1))
		    		l = mid;
		    	else
		    		r = mid - 1;
		    }
		    
		    if(pc1[l] <= i - k + 1)
		    	ans += (l + 1);
		}
	}

	cout << ans << endl;
}

signed main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	int t = 1;
	// cin >> t;
	while(t--)
	solve();
}