Balloons: [[10,16], [2,8], [1,6], [7,12]]. Sorted by end: [[1,6], [2,8], [7,12], [10,16]]. Arrow at position . Bursts [1,6] and [2,8] (both contain ). Remaining: [7,12], [10,16].
Arrow at position . Bursts [7,12] and [10,16] (both contain ). Remaining: none. Total arrows: . Notice [2,8] and [7,12] do not overlap (gap at 6-7), so they need separate arrows. The key is finding where one group ends and another begins. Each arrow represents a maximal group of overlapping balloons.