Problem Statement

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

Solution

class Solution:
    def search(self, nums: List[int], target: int) -> int:
        """
        0   1   2   3   4   5
        ----------------------
                        / target
        -1, 0,  3,  5,  9,  12
        ^a
                ^m
                            ^b
        """
        a, b = 0, len(nums) - 1

        while a <= b:
            m = (b + a) // 2

            if nums[m] == target:
                return m
            elif nums[m] < target:
                a = m + 1
            else:
                b = m - 1
        
        return -1

Leetcode 704 Binary Search