704. Binary Search.md

704. Binary Search

Question:

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].

Solution:

• Method 1: binary search

  class Solution {
      public int search(int[] nums, int target) {
          if(nums == null || nums.length == 0) return -1;
          return bs(nums, 0, nums.length - 1, target);
      }
      private int bs(int[] nums, int low, int high, int target){
          if(low > high) return -1;
          int mid = low + (high - low) / 2;
          if(nums[mid] == target) return mid;
          else if(nums[mid] > target) return bs(nums, low, mid - 1, target);
          else return bs(nums, mid + 1, high, target);
      }
  }