Codeforces 650B Image Preview 【二分】

题目链接：Codeforces 650B Image Preview

题意：给你n张图片，每次可以花费时间a移动到临近的图片。图片的位置分w和h，w的需要花时间b翻转位置。看一张照片花时间1，问你在时间t的前提下最多可以看多少照片。

思路：二分，对于长度o，分o种情况讨论，看是否满足<=t。

AC代码：

#include <iostream>#include <cstdio>#include <cstring>#include <cstdlib>#include <cmath>#include <algorithm>#include <vector>#include <queue>#include <map>#define CLR(a, b) memset(a, (b), sizeof(a))#define fi first#define se secondusing namespace std;typedef long long LL;typedef pair<int, int> pii;const int MAXN = 5*1e5+10;const int INF = 0x3f3f3f3f;void getmax(int &a, int b) {a = max(a, b); }void getmin(int &a, int b) {a = min(a, b); }char str[MAXN];int sum[MAXN];int n, a, b, t;int get(int x, int y) {    return min((x-1)*a*2 + (n-y+1)*a, (n-y+1)*a*2 + (x-1)*a); // s -> x -> s -> y or s -> y -> s -> x}bool judge(int o){    for(int i = 1; i <= o; i++) {        if(sum[i] + sum[n] - sum[n-o+i] + get(i, n-(o-i)+1) <= t) return true;    }    return false;}int main(){    cin >> n >> a >> b >> t;    cin >> str + 1; sum[0] = 0;    for(int i = 1; i <= n; i++)    {        int v;        if(str[i] == 'h') v = 1;        else v = b + 1;        sum[i] = sum[i-1] + v;        //cout << sum[i] << endl;    }    int l = 0, r = n; int ans = 0;    while(r >= l)    {        int mid = (l + r) >> 1;        if(judge(mid))        {            ans = mid;            l = mid + 1;        }        else            r = mid - 1;    }    cout << ans << endl;    return 0;}