UVA 11853 - Paintball(dfs）

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UVA 11853 - Paintball

题目链接

题意：就是给定一些圆，判断能否不经过这些圆从左边走到右边，如果可以要求起始和终止位置尽量往北，输出位置

思路：每次找一个超出上边界的圆dfs，如果能到下边界，就是隔断了肯定到达不了，如果隔断左右边界，就要更新答案的值

代码：

#include <cstdio>#include <cstring>#include <cmath>#include <cstdlib>#include <algorithm>using namespace std;const int N = 1005;int n, vis[N];struct Circle {int x, y, r;void read() {scanf("%d%d%d", &x, &y, &r);}} c[N];double ans1, ans2;bool can(Circle a, Circle b) {int dx = a.x - b.x;int dy = a.y - b.y;return dx * dx + dy * dy - (a.r + b.r) * (a.r + b.r) <= 0;}bool dfs(int u) {vis[u] = 1;if (c[u].y - c[u].r <= 0) return false;if (c[u].x - c[u].r <= 0) ans1 = min(ans1, c[u].y - sqrt(c[u].r * c[u].r - c[u].x * c[u].x));if (1000 - c[u].x - c[u].r <= 0) ans2 = min(ans2, c[u].y - sqrt(c[u].r * c[u].r - (1000 - c[u].x) * (1000 - c[u].x)));for (int i = 0; i < n; i++) {if (vis[i]) continue;if (can(c[u], c[i]))if (!dfs(i)) return false;}return true;}void solve() {ans1 = ans2 = 1000;for (int i = 0; i < n; i++) {if (!vis[i] && c[i].y + c[i].r >= 1000)if (!dfs(i)) {printf("IMPOSSIBLE\n");return;}}printf("0.00 %.2lf 1000.00 %.2lf\n", ans1, ans2);}int main() {while (~scanf("%d", &n)) {for (int i = 0; i < n; i++) {c[i].read();vis[i] = 0;}solve();}return 0;}

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