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.

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].

Solution

/**
 * @param {number[][]} points
 * @return {number}
 */
var findMinArrowShots = function (points) {
  // if the very left edge of the balloons are sorted (min to max)
  // then we only need to track the overlap and right edges
  points.sort((a, b) => a[0] - b[0]);

  // if there is at least one balloon
  // then a minimum of one dart is required

  let darts = 1;
  let currentOverlapR = points[0][1]; // right edge of previous overlap

  for (let i = 1; i < points.length; i++) {
    // if the current balloon's left edge is greater than the right
    // edge of our current overlap, so we need another dart
    if (points[i][0] > currentOverlapR) {
      darts++;
      currentOverlapR = points[i][1]; // reset the overlapping edge
      continue;
    }
    if (points[i][1] <= currentOverlapR) {
      // don't need to increment darts here due to overlap
      // but we do need to adjust where the right edge ends
      currentOverlapR = Math.min(currentOverlapR, points[i][1]);
    }
  }

  return darts;
};