452. Minimum Number of Arrows to Burst Balloons

Medium
There are some spherical balloons taped onto a flat wall that represents the XY-plane. The balloons are represented as a 2D integer array points where points[i] = [xstart, xend] denotes a balloon whose horizontal diameter stretches between xstart and xend. You do not know the exact y-coordinates of the balloons.
Arrows can be shot up directly vertically (in the positive y-direction) from different points along the x-axis. A balloon with xstart and xend is burst by an arrow shot at x if xstart <= x <= xend. There is no limit to the number of arrows that can be shot. A shot arrow keeps traveling up infinitely, bursting any balloons in its path.
Given the array points, return the minimum number of arrows that must be shot to burst all balloons.
Example 1:
Input: points = [[10,16],[2,8],[1,6],[7,12]]
Output: 2
Explanation: The balloons can be burst by 2 arrows:
- Shoot an arrow at x = 6, bursting the balloons [2,8] and [1,6].
- Shoot an arrow at x = 11, bursting the balloons [10,16] and [7,12].
Example 2:
Input: points = [[1,2],[3,4],[5,6],[7,8]]
Output: 4
Explanation: One arrow needs to be shot for each balloon for a total of 4 arrows.
Example 3:
Input: points = [[1,2],[2,3],[3,4],[4,5]]
Output: 2
Explanation: The balloons can be burst by 2 arrows:
- Shoot an arrow at x = 2, bursting the balloons [1,2] and [2,3].
- Shoot an arrow at x = 4, bursting the balloons [3,4] and [4,5].
Constraints:
  • 1 <= points.length <= 10^5
  • points[i].length == 2
  • -2^31 <= xstart < xend <= 2^31 - 1

解題

func findMinArrowShots(points [][]int) int {
sort.Slice(points, func(i, j int) bool {
return points[i][1] < points[j][1] // 根據氣球的右邊最排序,待會從左邊開始戳破
})
​
arrowPos := points[0][1]
ans := 1
for i:=1; i<len(points); i++ {
if (arrowPos >= points[i][0]) { //戳不到
continue
}
ans++ //多加一根針
arrowPos = points[i][1] //戳當下氣球的最後面
}
return ans
}