题目描述
统计一个数字在排序数组中出现的次数。
思路分析
为了让时间复杂度较低,考虑用二分查找。分别用两次二分查找,找到目标值第一次出现时的下标和最后一次出现时的下标。注意边界条件。(如:目标值只在数组的首或尾出现了1次)
# -*- coding:utf-8 -*-
class Solution:
def GetNumberOfK(self, data, k):
# write code here
# 用二分法分别找到第一个k和最后一个k
if not data:
return 0
start = 0
end = len(data) - 1
first_index = self.find_first(data, k, start, end)
last_index = self.find_last(data, k, start, end)
if first_index == -1 or last_index == -1:
return 0
return last_index - first_index + 1
def find_first(self, data, k, start, end):
mid = (start + end) // 2
if start > end:
return -1
if data[mid] == k:
#mid为表头或者mid左边的元素仍然等于k
if mid - 1 < start or data[mid-1] < k:
return mid
elif data[mid-1] == k:
return self.find_first(data, k, start, mid-1)
if data[mid] > k:
return self.find_first(data, k, start, mid-1)
if data[mid] < k:
return self.find_first(data, k, mid+1, end)
def find_last(self, data, k, start, end):
mid = (start + end) // 2
if start > end:
return -1
if data[mid] == k:
if mid + 1 > end or data[mid+1] > k:
return mid
elif data[mid+1] == k:
return self.find_last(data, k, mid+1, end)
if data[mid] > k:
return self.find_last(data, k, start, mid-1)
if data[mid] < k:
return self.find_last(data, k, mid+1, end)