算法：poj1066 宝藏猎人问题。

package practice;

import java.util.Scanner;

public class TreasureHunt {

    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        int[][] walls;
        float x, y;
        int doors = Integer.MAX_VALUE, temp1, temp2;
        int n = cin.nextInt();
        walls = new int[n][4];
        for (int j = 0; j < n; j++) {
            walls[j][0] = cin.nextInt();
            walls[j][1] = cin.nextInt();
            walls[j][2] = cin.nextInt();
            walls[j][3] = cin.nextInt();
        }
        x = cin.nextFloat();
        y = cin.nextFloat();
        for (int j = 0; j < n; j++) {
            temp1 = find(walls[j][0], walls[j][1], x, y, walls, j);
            temp2 = find(walls[j][2], walls[j][3], x, y, walls, j);
            doors = temp1 < temp2 ? (temp1 < doors ? temp1 : doors)
                    : (temp2 < doors ? temp2 : doors);
        }
        if (n == 0)
            doors = 0;
        doors++;
        System.out.println("Number of doors = " + doors);
    }

    private static int find(int x1, int y2, float x, float y, int[][] walls,
            int j) {
        int count = 0;
        for (int i = 0, len = walls.length; i < len; i++) {
            if (i == j)
                continue;
            if (isIntersect(x1, y2, x, y, walls[i]))
                count++;
        }
        return count;
    }

    /**
     * 
     * 跨立实验
     */
    private static boolean isIntersect(int startX, int startY, float endX,
            float endY, int[] wall) {
        if ((Math.max(startX, endX) >= Math.min(wall[0], wall[2]))
                && (Math.max(wall[0], wall[2]) >= Math.min(startX, endX))
                && (Math.max(startY, endY) >= Math.min(wall[1], wall[3]))
                && (Math.max(wall[1], wall[3]) >= Math.min(startY, endY))
                && (multiply(wall[0], wall[1], endX, endY, startX, startY)
                        * multiply(endX, endY, wall[2], wall[3], startX, startY) > 0)
                && (multiply(startX, startY, wall[2], wall[3], wall[0], wall[1])
                        * multiply(wall[2], wall[3], endX, endY, wall[0],
                                wall[1]) > 0))
            return true;
        else
            return false;
    }

    private static double multiply(float x1, float y1, float x2, float y2,
            float x3, float y3) {
        return ((x1 - x3) * (y2 - y3) - (x2 - x3) * (y1 - y3));
    }

}

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