codeforces 1084E

本文探讨了在给定字符串范围[s,t]内,构建字典树以选择k个字符串的问题,目标是最化前缀和不同的字符串数量。通过贴着边界s和t构建树,避免重复计数,实现高效求解。

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传送门:http://codeforces.com/contest/1084/problem/E

题意:给出两个字符串s,t,选出k个字符串,使得字典序在[s,t]内且前缀和不同的数量最多。

题解:题目是询问构建一颗字典树,最左边不小于s,最右边不大于t,且结点个数最多,那么我们只要左边贴着s走,右边贴着t走即可,也就是如果当前左边不能走a右边不能走b的情况,我们要减掉相应的方案数。

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
string a,b;
int main(){
	int n;
	ll k;
	scanf("%d%lld",&n,&k);
	cin>>a>>b;
	ll now=1,ans=0;
	if(k==1){
		printf("%d\n",n);
		return 0;
	}
	for(int i=0;i<n;i++){
		now*=2;
		if(a[i]!='a')now--;
		if(b[i]!='b')now--;
		now=min(now,k);
		ans+=now;
	}
	printf("%lld\n",ans);
	return 0;
}

 

### Codeforces 887E Problem Solution and Discussion The problem **887E - The Great Game** on Codeforces involves a strategic game between two players who take turns to perform operations under specific rules. To tackle this challenge effectively, understanding both dynamic programming (DP) techniques and bitwise manipulation is crucial. #### Dynamic Programming Approach One effective method to approach this problem utilizes DP with memoization. By defining `dp[i][j]` as the optimal result when starting from state `(i,j)` where `i` represents current position and `j` indicates some status flag related to previous moves: ```cpp #include <bits/stdc++.h> using namespace std; const int MAXN = ...; // Define based on constraints int dp[MAXN][2]; // Function to calculate minimum steps using top-down DP int minSteps(int pos, bool prevMoveType) { if (pos >= N) return 0; if (dp[pos][prevMoveType] != -1) return dp[pos][prevMoveType]; int res = INT_MAX; // Try all possible next positions and update 'res' for (...) { /* Logic here */ } dp[pos][prevMoveType] = res; return res; } ``` This code snippet outlines how one might structure a solution involving recursive calls combined with caching results through an array named `dp`. #### Bitwise Operations Insight Another critical aspect lies within efficiently handling large integers via bitwise operators instead of arithmetic ones whenever applicable. This optimization can significantly reduce computation time especially given tight limits often found in competitive coding challenges like those hosted by platforms such as Codeforces[^1]. For detailed discussions about similar problems or more insights into solving strategies specifically tailored towards contest preparation, visiting forums dedicated to algorithmic contests would be beneficial. Websites associated directly with Codeforces offer rich resources including editorials written after each round which provide comprehensive explanations alongside alternative approaches taken by successful contestants during live events. --related questions-- 1. What are common pitfalls encountered while implementing dynamic programming solutions? 2. How does bit manipulation improve performance in algorithms dealing with integer values? 3. Can you recommend any online communities focused on discussing competitive programming tactics? 4. Are there particular patterns that frequently appear across different levels of difficulty within Codeforces contests?
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