고정점 찾기.py

def binary_search(array,start,end):
  if start > end :
    return None
  mid = (start + end) // 2
 # target 없이 mid 와 array[mid] 값이 같다면 출력
  if array[mid] == mid:
    return mid
  elif array[mid] > mid:
    return binary_search(array,start,mid-1)
  else:
    return binary_search(array,mid+1,end)

n = int(input())
array = list(map(int,input().split()))

index = binary_search(array,0,n-1)
if index == None:
  print(-1)
else:
  print(index)