704. Binary Search

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LeetCode

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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:

• $1 <= nums.length <= 10^4$
• $-10^4 < nums[i], target < 10^4$
• All the integers in nums are unique.
• nums is sorted in ascending order.

两种写法

class Solution {
    public int search(int[] nums, int target) {
        int left = 0;
        int right = nums.length - 1;

        while (left <= right) {
            int mid = (left + right) / 2;
            int num = nums[mid];

            if (num == target) {
                return mid;
            } else if (num > target) {
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        }

        return -1;
    }
}

class Solution {
    public int search(int[] nums, int target) {
        int left = 0;
        int right = nums.length - 1;

        while (left + 1 < right) {
            int mid = (left + right) / 2;
            int num = nums[mid];

            if (num == target) {
                return mid;
            } else if (num > target) {
                right = mid;
            } else {
                left = mid;
            }
        }

        if (nums[left] == target) return left;
        if (nums[right] == target) return right;

        return -1;
    }
}