[Leet Code] Binary Search

Leetcode: https://leetcode.com/problems/binary-search/

Problem:

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:

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

Solution:

class Solution(object):
def search(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: int
"""
high = len(nums)-1
low = 0
return self.binary_search(nums, target, high, low)

def binary_search(self, nums, target, high, low):
print("high", high)
print("low", low)
if high >= low:
middle = (high + low) // 2
if target > nums[middle]:
return self.binary_search(nums, target, high, middle+1)
elif target < nums[middle]:
return self.binary_search(nums, target, middle-1, low)
else:
return middle
else:
return -1

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