题解：P14330 [JOI2021 预选赛 R2] 往返滑道 / Round Sugoroku

#include <bits/stdc++.h>
using namespace std;
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n, a;
    string s;
    cin >> n >> a >> s;
    
    vector<int> h;
    for (int i = 0; i < n; ++i)
        if (s[i] == '#') h.push_back(i + 1);  // 转为1-based位置
    
    int m = h.size();
    sort(h.begin(), h.end());
    long long t = 0;
    int p = a, d = 1, l = 0, r = n + 1;
    
    while (m) {
        if (d == 1) {  // 向右移动
            auto it = lower_bound(h.begin(), h.end(), p);
            int nh = (it != h.end()) ? *it : r + 1;
            int np = min(nh, r);
            
            t += np - p;
            p = np;
            
            if (p == r) d = -1;
            else { h.erase(it); m--; d = -1; }
        } else {  // 向左移动
            auto it = lower_bound(h.begin(), h.end(), p);
            int nh = (it != h.begin()) ? *prev(it) : l - 1;
            int np = max(nh, l);
            
            t += p - np;
            p = np;
            
            if (p == l) d = 1;
            else { h.erase(prev(it)); m--; d = 1; }
        }
    }
    
    cout << t << '\n';
    return 0;
}