classical binary search

Posted 2020-10-25 davidnyc

 

Given a target integer T and an integer array A sorted in ascending order, find the index i such that A[i] == T or return -1 if there is no such index.

Assumptions

• There can be duplicate elements in the array, and you can return any of the indices i such that A[i] == T.

Examples

• A = {1, 2, 3, 4, 5}, T = 3, return 2
• A = {1, 2, 3, 4, 5}, T = 6, return -1
• A = {1, 2, 2, 2, 3, 4}, T = 2, return 1 or 2 or 3

Corner Cases

• What if A is null or A is of zero length? We should return -1 in this case.

 

 1 public int binarySearch(int[] array, int target) {
 2     // Write your solution here.
 3     if(array == null || array.length == 0 ){
 4         return -1  ; 
 5     }
 6     int left = 0 ; 
 7     int right = array.length - 1 ; 
 8     //since we use mid + 1 or mid - 1, here we could have left = right condition
 9     while(left <= right){
10         int mid = left + (right-left)/2 ; 
11          if(array[mid] == target){
12           return mid ; 
13       } else if(array[mid]< target){
14           left = mid + 1 ; 
15       } else{
16           right = mid - 1 ; 
17       }
18     }
19     return -1;
20   }