[LeetCode] 704. Binary Search

Description

Given a sorted (in ascending order) integer array nums of n elements and a target value, write a function to search target in nums. If target exists, then return its index, otherwise return -1.

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

Note:

1. You may assume that all elements in nums are unique.
2. n will be in the range [1, 10000].
3. The value of each element in nums will be in the range [-9999, 9999].

Analyse

从一个list里找出一个数，不存在则返回-1

使用二分查找，每次将问题的规模减半

int search(vector<int>& nums, int target)
{
    int left = 0;
    int right = nums.size() - 1;
    int mid = (left + right) / 2;

    while (left <= right)
    {
        if (nums[mid] == target)
        {
            return mid;
        }
        else if (nums[mid] > target)
        {
            right = mid - 1;
        }
        else if (nums[mid] < target)
        {
            left = mid + 1;
        }
        mid = (left + right) / 2;
    }

    return -1;
}