LeetCode 452. Minimum Number of Arrows to Burst Balloons

452. Minimum Number of Arrows to Burst Balloons

There are a number of spherical balloons spread in two-dimensional space. For each balloon, provided input is the start and end coordinates of the horizontal diameter. Since it's horizontal, y-coordinates don't matter and hence the x-coordinates of start and end of the diameter suffice. Start is always smaller than end. There will be at most 104 balloons.

An arrow can be shot up exactly vertically from different points along the x-axis. A balloon with xstart and xend bursts by an arrow shot at x if xstart ≤ x ≤ xend. There is no limit to the number of arrows that can be shot. An arrow once shot keeps travelling up infinitely. The problem is to find the minimum number of arrows that must be shot to burst all balloons.

Example:

Input:
[[10,16], [2,8], [1,6], [7,12]]

Output:
2

Explanation:
One way is to shoot one arrow for example at x = 6 (bursting the balloons [2,8] and [1,6]) and another arrow at x = 11 (bursting the other two balloons).

解题思路：

1. 典型的贪心算法，把输入的气球按照Xend的大小，从小到大进行排序，这里我利用 ArrayList<int[]>来保存排序后的气球列表

2. 从第一个气球开始，它的Xend就是所有气球中最小的（因为已经排好序），记录为min，寻找后面所有Xstart小于min的气球，这样找到的气球，只需要一支x=min的箭就能够全部射破，把射破的气球全部remove，箭的数量加一，一次循环结束。一直循环到list为空，所有的气球全被刺破，返回count即可

代码如下：

public class Solution {
    public static int findMinArrowShots(int[][] points) {
        ArrayList<int[]> sortedList = sortList(points);
        return Count(sortedList);
        
    }


    public static ArrayList<int[]> sortList(int[][] points){
        ArrayList<int[]> list = new ArrayList<>();
        for(int i = 0;i < points.length;i++){            

                if(list.isEmpty()){
                    list.add(points[i]);
                }
                else{
                    int[] array = list.get(list.size() - 1);
                    if(points[i][1] > array[1]){
                        list.add(points[i]);
                        continue;
                    }
                    
                    for(int t = 0;t < list.size();t++){
                        array = list.get(t);
                        if(points[i][1] < array[1]){
                            list.add(t,points[i]);
                            break;
                        }

                    }
                }

            
        }
        return list;
    }

    public static int Count(ArrayList<int[]> sortedList){
        int count = 0;
        while(!sortedList.isEmpty()){
            int[] array = sortedList.get(0);
            int min = array[1];

            while(!sortedList.isEmpty()){
                if(sortedList.size() == 1){
                    sortedList.remove(0);
                    count++;
                    break;
                }
                int[] temp = sortedList.get(1);
                if(min >= temp[0]){
                    sortedList.remove(1);
                }
                else{
                    sortedList.remove(0);
                    count++;
                    break;
                }
            }
        }
        return count;
        
    }
}