1608. Special Array With X Elements Greater Than or Equal X

class Solution:
    def specialArray(self, A: List[int]) -> int:
        """ tc O(lgN) sc O(1)
        a is non-negtive => x range is [1,n], check edge case when x = 0, should return -1 
        """
        #A.sort() not necessary since x is correlated with size of A instead of value 
        n = len(A)
        l,r = 1,n # was 0,n+1 with l < r while loop 
        while l <= r: # here reduce extra work of checking l = r, if cnt qualify  
            mid = l + (r-l)//2 # check cases when r - l == 1 , mid = l 
            cnt = 0 
            for a in A:
                if a >= mid:
                    cnt += 1
            if cnt < mid: # mid is too big 
                r = mid -1
            elif cnt == mid:
                return mid
            else:  #cnt > mid :
                    l = mid + 1
        return  -1