Find Missing Term in Arithmetic Progression 等差数列缺失项

查找等差数列中的缺失项.

e.g.Input: arr[] = {2, 4, 8, 10, 12, 14} 

Output: 6

Input: arr[] = {1, 6, 11, 16, 21, 31};

Output: 26.

采用binary search. 若是arr[mid] - arr[left] == (mid - left) * diff, 说明missing 部分在mid右侧.

否则missing部分在包括mid的左侧.

Time complexity: O(logn). Space: O(1).

public class Main {

    public static void main(String[] args) {
        try{
            int [] arr1 = {2, 4, 8, 10, 12, 14};
            int [] arr2 = {1, 6, 11, 16, 21, 31};
            int [] arr3 = {1, 6};
            System.out.println(findMissing(arr1));
            System.out.println(findMissing(arr2));
            
            System.out.println(findMissing(arr3));
            
        }catch(IllegalArgumentException e){
            System.out.println(e.getMessage());
        }    
    }
    
    private static int findMissing(int [] arr) throws IllegalArgumentException{
        if(arr == null || arr.length < 3){
            throw new IllegalArgumentException("Invalid input!");
        }
        int len = arr.length;
        int l = 0;
        int r = len-1;
        int diff = (arr[r] - arr[l])/len;
        System.out.println(diff);
        while(l <= r){
            int mid = l+(r-l)/2;
            if(arr[mid] - arr[l] == (mid-l)*diff){
                l = mid+1;
            }else{
                r = mid;
            }
        }
        return arr[r] - diff;
    }
}