113 lines
3.2 KiB
C++
113 lines
3.2 KiB
C++
#include <iostream>
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#include <cstdio>
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#include <cstdlib>
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#include <algorithm>
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#include <functional>
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#include <vector>
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#include <map>
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#include <cstring>
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#include <string>
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#include <list>
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#include <set>
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#include <bitset>
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#include <cctype>
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#include <cmath>
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#include <ctime>
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#include <numeric>
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#include <utility>
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#include <stack>
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#include <queue>
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#include <deque>
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#include <iomanip>
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#include <cassert>
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#define pb push_back
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#define mp make_pair
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#define Maxn 1100
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#define fi first
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#define se second
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using namespace std;
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typedef pair<double, double> pii;
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const double eps=1e-8;
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const double inf=1e300;
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int n;
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double dp[Maxn][2];
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pii seg[Maxn][2];
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inline double getdis(pii a, pii b)
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{
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return sqrt((a.fi-b.fi)*(a.fi-b.fi)+(a.se-b.se)*(a.se-b.se));
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}
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inline int sign(double x)
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{
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return x<-eps?-1:x>eps;
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}
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pii intersert(pii A, pii B, double y)
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{
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double dx = A.fi - B.fi;
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double dy = A.se - B.se;
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return mp(A.fi + (y-A.se)*dx/dy, y);
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}
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bool leftturn(pii p, pii a, pii b)
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{
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double x1 = (a.fi-p.fi), y1 = (a.se-p.se);
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double x2 = (b.fi - p.fi), y2 = (b.se-p.se);
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return sign(x1*y2-x2*y1)>0;
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}
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bool noRightTurn(pii p, pii a, pii b)
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{
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double x1 = (a.fi-p.fi), y1 = (a.se-p.se);
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double x2 = (b.fi - p.fi), y2 = (b.se-p.se);
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return sign(x1*y2 - x2*y1)>=0;
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}
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int main()
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{
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double a, b, c;
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while (~scanf("%d", &n), n)
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{
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scanf("%lf%lf", &a, &b);
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seg[0][0] = mp(a, b);
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for (int i=1; i<=n; i++)
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{
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scanf("%lf%lf%lf", &c, &a, &b);
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seg[i][0] = mp(a, c);
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seg[i][1] = mp(b, c);
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}
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dp[n][0] = dp[n][1] = 0;
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int j;
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for (int i=n-1; i>=0; i--)
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{
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for (int side=0; side<2; side++)
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{
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dp[i][side] = inf;
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pair<pii,pii> next = mp(seg[i+1][0], seg[i+1][1]);
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pii pt = seg[i][side];
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for (j=i+1; j<=n; j++)
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{
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if (leftturn(pt, seg[j][1], next.fi) || leftturn(pt, next.se, seg[j][0])) break;
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if (noRightTurn(pt, next.fi, seg[j][0]))
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{
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next.fi = seg[j][0];
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dp[i][side] = min(dp[i][side], getdis(pt, next.fi)+dp[j][0]);
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}
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if (noRightTurn(pt, seg[j][1], next.se))
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{
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next.se = seg[j][1];
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dp[i][side] = min(dp[i][side], getdis(pt, next.se)+dp[j][1]);
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}
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}
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if (j > n)
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{
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double y = seg[n][0].se;
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next.fi = intersert(pt, next.fi,y);
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next.se = intersert(pt, next.se, y);
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if (pt.fi < next.fi.fi) dp[i][side] = min(dp[i][side], getdis(pt, next.fi));
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else if (pt.fi > next.se.fi) dp[i][side] = min(dp[i][side], getdis(pt, next.se));
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else dp[i][side] = min(dp[i][side], pt.se - y);
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}
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if (i == 0) break;
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}
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}
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printf("%.10f\n", dp[0][0]);
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}
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return 0;
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}
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