[leetcode] 704. Binary Search

题目

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.

Example 1:

Input: nums = [-1,0,3,5,9,12], target = 9
Output: 4
Explanation: 9 exists in nums and its index is 4

Example 2:

Input: nums = [-1,0,3,5,9,12], target = 2
Output: -1
Explanation: 2 does not exist in nums so return -1

Constraints:

• 1 <= nums.length <= 104
• -104 < nums[i], target < 104
• All the integers in nums are unique.
• nums is sorted in ascending order.

思路

双指针框定数组的边界，取数组位于双指针的中间值与目标值判断大小，如果中间值比目标值小，那么说明目标值位于中间值和右指针的中间，因此将左指针移动至中间值的位置，反之将右指针移动至中间值的位置。如此反复直至在数组中找出目标值的位置，或双指针重合，未找到目标值。

代码

python版本：

class Solution:
    def search(self, nums: List[int], target: int) -> int:
        l,r=0,len(nums)
        while l<r:
            m=(l+r)//2
            if target>nums[m]:
                l=m+1
            elif target<nums[m]:
                r=m
            else:
                return m
        return -1