Python世界：力扣题704二分查找

任务背景

问题来自力扣题目704：Binary Search，大意如下：

Given an array of integers nums which is sorted in ascending order, and an integer target, write a function to search target in nums. If target exists, then return its index. Otherwise, return -1.

You must write an algorithm with O(log n) runtime complexity.

翻译下，需求是：对有序数组进行查找指定数字，若有返回索引，若无返回-1.

思路分析

重温下二分写法，思路很简单，发现值大的下移上界，发现值小的上移下界，直到上下界重合。

要注意的是无target时，mid的偏移问题。

代码实现

class Solution(object):
    def search(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: int
        """
        # range: [low, high)
        low = 0
        high = len(nums)
        while (low < high):
            mid = low + (high - low) // 2
            if nums[mid] < target:
                low = mid + 1
            elif nums[mid] > target:
                high = mid
            else:
                return mid
        # not found
        return -1

   
# test
nums = [-1, 0, 3, 5, 9, 12]
target = 9

# nums = [-1,0,3,5,9,12]
# target = 2

sol = Solution()
res = sol.search(nums, target)
print(res)

测试套件

# 导入单元测试
import unittest

# 编写测试套
class TestSol(unittest.TestCase):
    # 不在数组中
    def test_special1(self):
        nums = [-1, 0, 3, 5, 9, 12]
        target = 2
        ret = -1
        sol = Solution()
        self.assertEqual(sol.search(nums, target), ret)

    # 下边界
    def test_special2(self):
        nums = [-1, 0, 3, 5, 9, 12]
        target = -1
        ret = 0
        sol = Solution()
        self.assertEqual(sol.search(nums, target), ret)

    # 上边界
    def test_special3(self):
        nums = [-1, 0, 3, 5, 9, 12]
        target = 12
        ret = 5
        sol = Solution()
        self.assertEqual(sol.search(nums, target), ret)

    def test_common1(self):
        nums = [-1, 0, 3, 5, 9, 12]
        target = 5
        ret = 3
        sol = Solution()
        self.assertEqual(sol.search(nums, target), ret)

    def test_common2(self):
        nums = [-1, 0, 3, 5, 9, 12]
        target = 9
        ret = 4
        sol = Solution()
        self.assertEqual(sol.search(nums, target), ret)


# 含测试套版本主调
if __name__ == '__main__':
    print('start!')
    unittest.main() # 启动单元测试
    print('done!')

本文小结

二分核心：索引偏移存乎一心。

可进一步思考若有重复值时，如何找到最小重复索引或最大重复索引。