class Solution(object):
    def hIndex(self, citations):
        """
        :type citations: List[int]
        :rtype: int
        """
        length=len(citations)
        hi=length-1
        lo=0
        #find i such that citations[i]>=len(citations)-i
        while lo<=hi:
            mid=lo+(hi-lo)/2
            if citations[mid]==length-mid:
                return length-mid
            elif citations[mid]<length-mid:
                lo=mid+1
            else:
                hi=mid-1
        return length-lo